Marine Biological Laboratory Library
Woods Hole, Mass.
ESTATE OF HERBERT W. RAND
January 9, 1964
SMELL, TASTE, AND ALLIED SENSES
IN THE VERTEBRATES
MONOGRAPHS ON EXPERIMENTAL
FORCED MOVEMENTS, TROPISMS, AND ANIMAL
By JACQUES LOEB. Rockefeller Institute
THE ELEMENTARY NERVOUS SYSTEM
By G. H. PARKER, Harvard University
THE PHYSICAL BASIS OF HEREDITY
By T. H. MORGAN. Columbia University
INBREEDING AND OUTBREEDING: THEIR GENETIC
AND SOCIOLOGICAL SIGNIFICANCE
By B. M. EAST and D. F. JONES. Bussey Institution. Harvard University
THE NATURE OF ANIMAL LIGHT
By E. N. HARVEY. Princeton University
SMELL, TASTE AND ALLIED SENSES IN THE
By G. H. PARKER. Harvard University
BIOLOGY OF DEATH
By R. PEARL. Johns Hopkins University
PURE LINE INHERITANCE
By H. S. JENNINGS. Johns Hopkins University
LOCALIZATION OF MORPHOGENETIC SUBSTANCES
IN THE EGG
By E. G. CONKLIN, Princeton University
By R. G. HARRISON. Yale University
INJURY, RECOVERY AND DEATH IN RELATION TO
CONDUCTIVITY AND PERMEABILITY
By W. J. V. OSTERHOUT. Harvard University
THE EQUILIBRIUM BETWEEN ACIDS AND BASES IN
ORGANISM AND ENVIRONMENT
By L. J. HENDERSON. Harvard University
CHEMICAL BASIS OF GROWTH
By T. B. ROBERTSON. University of Toronto
COORDINATION IN LOCOMOTION
By A. R. MOORE. Rutgers College
OTHERS WILL FOLLOW
MONOGRAPHS ON EXPERIMENTAL BIOLOGY
SMELL, TASTE, AND ALLIED
SENSES IN THE VERTEBRATES
G. H. PARKER, Sc.D.
PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY
PHILADELPHIA AND LONDON
J. B. LIPPINCOTT COMPANY
COPYRIGHT. Ip22. BY J. B. LIPPINCOTT COMPANY
Electrotypcd and Printed by J. B. Li ppincott Company
The Washington Square Press, Philadelphia, U. S. A.
THE rapid increase of specialization makes it im-
possible for one author to cover satisfactorily the whole
field of modern Biology. This situation, which exists in
all the sciences, has induced English authors to issue
series of monographs in Biochemistry, Physiology, and
Physics. A number of American biologists have decided
to provide the same opportunity for the study of
Biology, which not long ago was purely descriptive
and speculative, has begun to adopt the methods of the
exact sciences, recognizing that for permanent progress
not only experiments are required but quantitative experi-
ments. It will be the purpose of this series of monographs
to emphasize and further as much as possible this develop-
ment of Biology.
Experimental Biology and General Physiology are one
and the same science, in method as well as content, since
both aim at explaining life from the physico-chemical
constitution of living matter. The series of monographs
on Experimental Biology will therefore include the field
of traditional General Physiology.
T. H. MORGAN,
W. J. V. OSTERHOUT.
SENSE organs have always excited general interest,
for they are the means of approach to the human mind.
Without them our intellectual life would be a blank. The
deaf and the blind show how serious is the loss of even a
single set of these organs.
Although the ear and the eye have commonly received
most attention, the other sense organs, such as those of
smell and of taste, are in reality equally worthy of con-
sideration. These organs are of first significance in
warning us of untoward conditions that may exist about
us particularly in relation to our food. But they not only
serve us in this protective way, they are also of the utmost
importance in initiating that chain of events which cul-
minates in successful nutrition. Through their action the
secretion of the digestive juices and other like operations,
so essential to the proper treatment of the food, are
started and furthered in the alimentary canal. Thus
their activities, though less associated with our mental
states than are those of the ear and of the eye, are never-
theless so essential to our organic well-being that they
are in reality quite as necessary to us as the so-called
Smell and taste, together with certain other senses not
so well known, form a more or less natural group in which
there is a certain amount of functional interrelation and
genetic connection, and it is from this standpoint that
these senses will be considered in the following pages.
They will thus illustrate in a way principles common to
8 AUTHOR'S PREFACE
other groups of sense organs, and these principles will be
found to be of an essentially dynamic character as con-
trasted with the older conceptions in which function has
been brought into relation less intimately with structure.
The author is greatly indebted to the editors of this
series of monographs for many suggestions that have led
to improvements in the text. He is also under obligations
to his wife for a careful revision of the manuscript. He
wishes to extend his thanks to numerous persons who
have permitted him to copy and use figures contained in
their publications. In all such instances the sources of
such figures are acknowledged in the text. Where a
figure is given without reference, it is an original. The
drawings for all figures were made by Mr. E. N. Fisher.
G. H. P.
Harvard University, Cambridge, Mass.
I. NATURE OF SENSE ORGANS 13
II. ANATOMY OF THE OLFACTORY ORGAN 23
III. PHYSIOLOGY OF OLFACTION 42
IV. VOMERO-NASAL ORGAN OR ORGAN OF JACOBSON 92
V. THE COMMON CHEMICAL SENSE 102
VI. ANATOMY OF THE GUSTATORY ORGAN 110
VII. PHYSIOLOGY OF GUSTATION 132
VIII. INTERRELATION OF THE CHEMICAL SENSES. . 167
INDEX . 187
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1. Diagram of the Lateral Wall of the Right Nasal Cavity of Man . . 24
2. Diagram of a Transverse Section of the Right Nasal Cavity of Man 25
3. Respiratory Epithelium from the Nasal Cavity of a Young Pig. ... 27
4. Olfactory Cleft of Man 28
5. Olfactory Epithelium from a Pig Embryo 29
6. Olfactory Epithelium from a Young Mouse 30
7. Isolated Olfactory Cells and Sustentacular Cells from Man 31
8. Isolated Olfactory Cell and Sustentacular Cell from a Frog 32
9. Olfactory Cell of a Pike Showing Flagellum 33
10. Olfactory Epithelium from a Chick Embryo 36
11. Ventral View of the Head of a Shark (Scyllium) 38
12. Diagram of the Right Nasal Cavity of Man Showing the Direction
of the Inspired Air Currents 46
13. Simple Rubber Olfactometer 50
14. Double Olfactometer 51
15. Ventral View of the Head of a Hammer-head Shark 66
16. Curves of Olfactory Exhaustion 71
17. Olfactory Prism 75
18. Generalized Diagrams of the Molecular Structure of Classes
of Aromatic Bodies (Olfactory Stimuli) 80
19. Head of Human Embryo showing Vomero-nasal Pore 93
20. Diagram of the Median Face of the Left Nasal Cavity of Man 94
21. Transverse Section of the Snout of a Young Frog 95
22. Transverse Section of the Head of a Snake Embryo 96
23. Transverse Section of the Nasal Septum of a Young Cat 97
24. Epithelium from the Vomero-nasal Organ of the Sheep 98
25. Dorsal View of the Human Tongue 112
26. Vertical Section of a Fungiforra Papilla 113
27. Vertical Section of a Vallate Papilla 114
28. Lateral View of a Catfish Showing Gustatory Branches of the
Facial Nerve 116
29. A Simple Taste-bud 117
30. A Compound Taste-bud 118
31. Taste-buds of the Rabbit 121
32. Taste-buds of the Cat 122
33. Taste-buds of the European Barbel 124
34. Diagram of the Human Tongue Showing Innervation 125
35. Diagram of the Possible Paths of the Gustatory Nerves in Man. . . . 126
36. Diagrams of the Human Tongue Showing the Distribution of the
Four Tastes 149
37. Diagrams of the Receptor Systems of the Vertebrate Chemoreceptors 181
SMELL, TASTE, AND ALLIED
SENSES IN THE VERTEBRATES
NATURE OF SENSE ORGANS.
Contents. 1. Older Conception of Sense Organs. 2.
Modified View due to Theory of Reflex Action. 3. The
Genesis of Receptors. 4. Bibliography.
1. OLDER Conception of Sense Organs. In the con-
ventional text-book, sense organs are commonly looked
upon as structures that supply the brain with those nerv-
ous impressions from which the mental life of the indi-
vidual is built. During normal activity these organs are
incessantly in operation and flood the central apparatus
with a stream of impulses by which are carried to us evi-
dences of the multitudinous alterations of the environ-
ment. Through the ear and the eye pass continuous
streams of change by which we adjust ourselves not only
to the immediate material world about us but to the
world of ideas whose elements are spoken and writ-
Sense organs from a structural standpoint are organs
whose cells are so specialized that they are subject to stim-
ulation by only a particular category of external changes.
As Keith Lucas has expressed it, sense cells approximate
a unifunctional state. The changes by which they are
brought into action form rather homogeneous groups of
14 SMELL, TASTE, ALLIED SENSES
environmental alterations. Thus the chemical changes of
the surroundings affect the organs of smell and of taste,
the pressure changes those of touch and hearing, and al-
terations in the radiant energy those of sight. These
natural groups of environmental changes have been des-
ignated as homologous, or, better, adequate stimuli for
the sense organ that they activate. Such organs are ordi-
narily arranged under five heads each with an adequate
stimulus and productive of a special sensation ; they are
the organs of smell, taste, touch, hearing, and sight.
Experience has also shown that when in a given per-
son a sense organ exhibits complete congenital incapacity,
such an individual lacks certain mental elements that can
never in reality be made good to him by the activity of the
remaining parts. A state of this kind implies a certain
mental deficiency in the given individual. If a person has
been blind from birth, no amount of description can
supply to him the sensations of the wealth of color that the
external world holds for the normal man. Where blind-
ness is an acquired defect, the rememberance of the
former color sensations as compared with the present
deprivation, makes the state of deficiency still more pro-
nounced. And in those rare cases where there is a
unilateral defect in color vision with sight otherwise unim-
paired, the subject can contrast most vividly the state of
deficiency with that of normal completeness. Such con-
ditions, which are known to occur not only in sight but in
the other senses as well, have had a most profound influ-
ence on the interpretations that naturalists have placed
upon the states presented by the lower animals.
It has been commonly assumed, and with no small
show of reason, that where an animal is found to possess
NATURE OF SENSE ORGANS 15
an eye or an ear, for instance, it should be accredited with
all the central nervous activities, sensations and the like,
that accompany such an organ in man, qualified only by
the degree of development to which the particular organ
in the given animal has arrived. Conclusions based
upon such a course of reasoning were commonly ad-
mitted as valid by the workers of a few decades ago
(Lubbock, 1882; Graber, 1884) and the text-books of that
period in dealing with the sense organs of the lower
animals discuss these parts ordinarily under the conven-
tional five heads of the older human physiology ( Jourdan,
1889). From this standpoint one of the lower animals is
like a defective human being in that its full sensory ac-
tivity falls short of that of the normal man. Or it may be
compared to a person whose sensory development is un-
symmetrical and whose relations with the surroundings
have come to be predominant through a limited number
of sensory channels rather than through all.
It is likewise perfectly clear that a given animal, whose
organization in general may be simpler than that of man,
may nevertheless excede him in a particular sensory
capacity and in this respect at least stand above him. It is
commonly admitted that the dog far outruns man in the
keeness of Ms sense of smell and it has long been known
that cats hear tones of a pitch much too high for the human
ear. These and other like examples show that though the
senses of the lower animals are in general less efficient
than those of man, the reverse is occasionally true.
Moreover among some of the lower forms, sense or-
gans have been discovered that are not represented in
man. Thus fishes possess, in addition to the five classes of
human sense organs, the so-called lateral-line organs.
16 SMELL, TASTE, ALLIED SENSES
Here then must be a wholly novel set of sensory relations.
As to the sensations arising from these organs man can
form no direct conception, for they are entirely outside the
range of his experience. Hence Leydig, the discoverer of
the sensory nature of these parts, wrote of them as organs
of a sixth sense. Thus to the older workers the senses of
the lower animals were like those of a human being that
had suffered either curtailment or expansion even to the
extent of excluding or including whole categories of
stimuli. But quite aside from the question of the number
and variety of these parts, is the opinion held by most of
the early workers that the sense organs of the lower ani-
mals are primarily concerned with providing the brain
or corresponding structure of the given creature with that
body of sensation which was supposed to represent all the
significant changes in the effective environment.
2. Modified View due to Theory of Reflex Action.
The belief that sense organs were chiefly concerned with
providing the brain with the elements of which the mental
life is composed suffered an important limitation from the
work of the physiologist. This limitation arose from the
development of the idea of reflex action. Originating
about the time of Descartes in the seventeenth century, the
conception of the reflex action grew in time into a most
important principle for the interpretation of nervous
operations. It was at first applied to that form of
nervous activity whose outcome is fairly constant and in
a way mechanical in that it is unassociated with conscious-
ness, but it was gradually extended to include those per-
formances in which consciousness is involved and at
present it commonly refers to any chain of nervous
activity in which a sensory stimulation produces an im-
NATURE OF SENSE ORGANS 17
pulse that, after passage through the central nervous
organs, results in action.
From the beginning many reflexes were believed to be
unassociated with consciousness and though this view was
subsequently combated and the idea of the reflex extended
to nervous operations that included an obvious sensa-
tional element, it nevertheless remained true that a host
of reflex operations could be pointed out that were with-
out representation in consciousness. Thus the impulses
that flow from the vestibular portion of the human ear
and that are of the utmost importance in maintaining
equilibrium provoke no obvious sensations and the vast
flux of afferent nerve action that moves from the mus-
cle to the spinal cord and that is so essential to the
coordination of bodily movements, runs its course without
exciting sensation. These and many like instances have
made it clear that the reflex, even in the most special ap-
plication of the term may as often be unassociated with
sensation as associated with it.
As the first step in every reflex is the excitation of a
sense organ and as many reflexes are unassociated with
consciousness, it must be admitted that sense organs, not-
withstanding the name, are not always necessarily con-
cerned with sensations. Many certainly have nothing
whatever to do with such central nervous states. Thus
it is doubtful if the normal activity of the sensory endings
in our muscles and tendons is ever productive of sensation.
In consequence of this condition a reasonable objection
was raised to the term sense organ and it was proposed by
Bethe (1897) to use in place of it the word receptor.
Although the theoretic force of this objection has not
always carried conviction, the term receptor has come into
18 SMELL,jTASTE, ALLIED SENSES
common use and the emphasis that it places on the organs
to which it is applied as receivers of environmental change
rather than as originators of impulses to sensation is
certainly a step in the right direction.
Human receptors belong to one or other of two classes.
Either they are concerned purely and simply with the
excitation of reflex acts and take no part in the pro-
duction of sensations, in which case they may be called
activators, or they are at the same time effective in
arousing sensations, the elements of the intellectual life
and hence may be appropriately termed sense organs.
All receptors belong to either one or the other of these
classes though in some instances a certain degree of
temporary vacillation occurs. Hence it may be that these
classes exemplify in a way two receptive functions, one
of which predominates in one class and the other in the
other. How these functions are related can best be
gathered from the genetic history of receptors.
3. The Genesis of Receptors. Eeceptors such as the eye
and the ear, the organs of smell and taste, and the more
diffuse sensory equipment of the skin, are found in all
the more complex animals. They abound in the verte-
brates, the mollusks,the arthropods, and to a less extent in
the worms. They may be said to occur even in the coe-
lenterates, as, for instance, among the jelly fishes, though
in the majority of these animals the receptors present a
diffuse condition more like that seen in the vertebrate
skin than in the vertebrate eye or ear. This diffuse state
seems to be characteristic of the receptors in the simpler
sessile invertebrates. The more complex animals such as
are capable of active locomotion exhibit almost invari-
ably specialized types of organs.
NATURE OF SENSE ORGANS 19
So far as the neuromuscular system of the inverte-
brates is concerned, forms as low in the scale as the annelid
worms appear to possess all the elements of the corre-
sponding system in the vertebrates. Such worms may
have specialized receptors, eyes and the like, often of a
highly complex structure. They possess a well-differ-
entiated central nervous system as represented in their
so-called brain and ventral ganglionic chain. Finally,
they have an abundant variety of specialized effectors
in their various muscles, glands, and luminous organs.
Their receptors, central nervous organs, and muscles are
so related that reflexes can be demonstrated on them as
readily as on vertebrate preparations. In other words,
they possess in completeness, though in simple form, a
working neuromuscular mechanism essentially like that
of the higher animals.
When, however, an examination of such forms as the
ccelenterates is made, it is found that the coral animals,
the sea-anemones, the hydroids, and the like, possess
scarcely any trace of a central nervous apparatus. In
these animals fairly well specialized sensory surfaces
occur, whose nervous prolongations connect either imme-
diately with the subjacent musculature or give rise to a
nerve-net which in turn connects with the contractile ele-
ments. Thus the receptor is applied to the muscle very
directly and without the intervention of a central organ.
Such an arrangement allows of simple reflexes, for, when
the receptive surface is stimulated, the animal responds
at once by an appropriate muscular movement. Thus if
meat juice is discharged on the tentacles of a sea-anemone,
these organs carry out vermiculate movements and the
gullet opens; or if the pedal edge of the column is touched,
20 SMELL, TASTE, ALLIED SENSES
the whole animal contracts. The fact that meat juice
will not excite the pedal edge of the column and that a
touch applied to the tentacles is seldom followed by more
than a slight local activity shows that the external surface
of the sea-anemone, though generally receptive, is locally
specialized. As a matter of fact this surface in degree of
differentiation stands between a diffuse receptive surface,
such as the vertebrate skin, and a specialized organ like
the eye or the ear.
In the literal sense of the word the outer surface of a
sea-anemone is not sensory though abundantly receptive.
There is no reason to suppose that the receptive areas of
these animals are concerned with initiating impulses to
sensation. They connect very directly with muscles and
serve quite obviously as trigger-like organs by which the
muscle is set in action. A careful examination of the
activities of sea-anemones has failed to reveal any evi-
dence, such as can be produced from the more complex
animals, to show that these simple creatures possess
central nervous functions. Such functions apparently
have no part in their organized performances. Hence
their receptors have nothing whatever to do with initia-
ting impulses to sensation, but are limited in their action
to the excitation of the muscles after the type of the most
mechanical reflex. The presence in ccelentrates of eye
spots, olfactory pits, statocysts and other such special
receptors is, therefore, no indication that these animals are
endowed with corresponding sensations, as many of the
older workers believed, but this condition merely shows
that their possessors are especially open to a particular
stimulus. An eye spot does not mean that the animal pos-
sesses sight, but that it is readily excited to action by light.
NATURE OF SENSE ORGANS 21
Thus of the two functions that have been attributed to
receptors, the capacity to excite action and the ability to
initiate impulses for sensation, the former is much the
more widely distributed of the two and is without question
the more primitive.
Since sponges are known to possess muscles but are
devoid of nervous tissue, it is probable that they represent
a type of organization which in point of time preceded
that in which the nervous elements arose. So far as can
be judged these elements originated in connection with the
previously differentiated muscle and as a special means
of exciting it to contraction. This earliest nervous mate-
rial must have been, therefore, essentially receptive in
character and must have served as the source of the more
obvious receptors of specialized types. Thus receptors
must be regarded as the original form of nervous struc-
ture, concerned in the beginning with the simple excita-
tion of muscle (activators) and subsequently involved,
after the development of the central organs, with that
supply of impulses which yields the elements of the intel-
lectual life (sense organs).
The extent to which a natural group of receptors may
undergo differentiation and yet maintain a striking degree
of mutual interdependence can nowhere be better illus-
trated than with the chemical receptors, the organs of
smell and of taste. It is from this standpoint that the
structure and function of these receptors will be con-
sidered in the following chapters.
BEER, T., A. BETHE, und J. VON UEXKULL. 1899. Vorschlage zu einer
objektivierenden Nomenklatur in der Physiologic des Nervensy steins.
Biol. Centralbl, Bd. 19, pp. 517-521.
22 SMELL, TASTE, ALLIED SENSES
BETHE, A. 1897. Das Nervensystem von Carcinus maenas. Arch. mik.
Anat., Bd. 50, pp. 460-546.
GBABEB. V. 1884. Grundlinien zur Erforschung des Helligkeits- und
Farbensinnes der Tiere. Prag & Leipzig, 322 pp.
JOURDAJST, E. 1889. Les sens chez les animaux inferieurs. Paris, 314 pp.
LUBBOCK, J. 1882. Ants, Bees, and Wasps. New York, 448 pp.
PABKEB, G. H. 1910. The Reactions of Sponges, with a Consideration of the
Origin of the Nervous System. Jour. Exp. Zool., vol., 8, pp. 1-41.
PABKEB, G. R. 1917. The Sources of Nervous Activity. Science, vol. 45,
PABKEB, G. H. 1919. The Elementary Nervous System. Philadelphia,
ANATOMY OF THE OLFACTORY ORGAN.
Contents. 1. Nasal Cavities in Man. 2. Nasal Mem-
branes. 3. Olfactory Epithelium. 4. Intermediate Zone.
5. Polymorphic Cells. 6. Sense Buds. 7. Free-nerve
Endings. 8. Development of Olfactory Nerve. 9. Com-
parative Anatomy of Olfactory Organs. 10. Bibliography.
1. NASAL Cavities in Man. In man the olfactory
organs are paired and are situated one in each nasal
cavity. Each of these cavities possesses an external
opening, the anterior nans, and an internal one, the
posterior naris or choana, which communicates with the
pharynx. (Fig. 1). The two nasal cavities are separated
by the nasal septum, a partly bony, partly cartilaginous
wall, which forms a smooth median partition between
them. The lateral walls of these cavities are thrown into
a series of more or less horizontal folds, the nasal conchas.
These are commonly three in number for each cavity
though in some instances only two are present and in
others a fourth, fifth or even a sixth can be discerned.
Of the three conchas usually present the most ventral one,
the inferior concha, is the largest and extends through
much of the length of the cavity in a direction approxi-
mately parallel to its floor. Immediately above the inferior
concha is the somewhat smaller middle concha which
is followed by the still smaller superior concha. When
only two conchas are present, they are the inferior and the
24 SMELL, TASTE, ALLIED SENSES
middle, the superior being absent. When a fourth concha
is to be seen, it is found above and behind the superior.
It has been designated the first supreme concha and it
Fio. 1. Diagram of the lateral wall of the right nasal cavity of man. I, Inferior concha;
2, middle concha; 3, superior concha; 4, first supreme concha; 5, second supreme concha; the
apertures numbered C to 10 arc covered from sight by the conchro, but their positions are
indicated by vertical lining; 6, aperture of the nnsolacrimal duct opening into the inferior
meatus; 7, opening of the maxillary sinus (middle meatus); 8, opening of the frontal sinus
(middle meatus); 'J, and 10, openings of the ethmoid cells, 9. into the middle meatus, 10,
into the superior meatus; 11, opening of the Kustachian tube; 12, vestibule; 13, atrium; 14,
choana; 15, frontal sinus; 10, sphenoidal sinus whose opening is indicated by an arrow; 17,
olfactory region whose limits are marked by the dotted line. The vertical dotted line shows
the plane of section from which Fig. 2 was drawn.
may be followed by a second or even, a third supreme
concha. According to Schaeffer(1920), the first supreme
concha is to be observed in about 60 per cent of all adult
The three conchae ordinarily present project from
ANATOMY OF THE OLFACTORY ORGAN
the lateral wall of each nasal chamber into its cavity and
partly divide that cavity into three approximately hori-
zontal passages: the inferior meatus under the inferior
concha, the middle meatus under the middle concha and
the superior meatus under the superior concha. (Fig. 2).
The external naris leads at once
to the first chamber of the nose,
the vestibule, which connects
almost directly with the inferior
meatus, less directly with the su-
perior meatus and through the
so-called atrium with the middle
meatus. Between the median sep-
tum of the nose and the laterally
situated conchas is a considerable
space known as the common
meatus. Dorsally this space is
continuous with a narrow slit
lying between the superior concha
and the septum and called the
olfactory cleft. All' these pas-
sages and spaces communicate
more or less directly and freely
through the posterior naris or
choana with the pharynx.
In the bones about the nose in man are large paired air-
spaces or sinuses that communicate with the exterior
through the nasal cavity. These spaces, which have been
very fully described by Schaeffer (1916), are of consid-
erable size and are lined with a mucous epithelium con-
tinuous with that of the nose. They are somewhat variable
in number and connections and yet they fall more or less
Fio. 2. Diagram of a trans-
verse section of the right nasal
cavity in man made at the plane
indicated by the vertical dotted
line in Fig. 1. 1, inferior concha;
2, middle concha ; 3, superior con-
cha; 4, nasal septum; 5, inferior
meatus; 6, middle meatus; 7,
superior meatus; 8, common
meatus; 9, olfactory cleft (left
side); 10, ethmoid cells; 11,
26 SMELL, TASTE, ALLIED SENSES
naturally into four sets, the maxillary, frontal, and sphe-
noidal sinuses and the ethmoidal cells. Each maxillary
sinus is a large space in the maxillary bone above the
teeth. It opens by a considerable slit into the anterior part
of the middle meatus. (Figs. 1 and 2). The frontal sinus,in
the frontal bone also opens into the middle meatus at a
point above and anterior to the opening of the maxillary
sinus. Each sphenoidal sinus opens into the posterior end
of the appropriate olfactory cleft in a region known as the
spheno-ethmoidal recess. The remaining accessory nasal
spaces, the ethmoid cells, are more or less variable ; some
of them open into the middle meatus by several apertures
well above the slit for the maxillary sinus. Others open,
more commonly by a single aperture, into the superior
meatus. In addition to these various openings, the naso-
lacrimal duct, by which the lacrimal secretions from the
eye are carried to the nasal cavity, opens between a
pair of lips on the lateral wall of the inferior meatus near
its anterior extremity,
2. Nasal Membranes. The nasal vestibule is lined
with a delicate continuation of the outer skin. The walls
of the deeper part of the nasal cavity are covered with a
mucous membrane which is divisible into two regions, the
restricted olfactory region in the dorsal part of the cavity
and the much more extended respiratory region embrac-
ing the remainder of the cavity.
The mucous membrane of the respiratory region is
reddish in color and consists of a pseudo-stratified epi-
thelium containing ciliated cells and basal cells backed up
by a well developed tunica propria. (Fig. 3.) The cilia
of this region lash towards the choana. The secretion
covering the surface of the epithelium comes from numer-
ANATOMY OF THE OLFACTORY ORGAN
ous branched alveolo-tubular glands which contain both
mucous and serous cells.
The conchae of the respiratory region have long been
known to be extremely vascular and to be possessed of a
structure like that of erectile tissue. This is especially
true of their edges. They can be
excited through reflex channels to
considerable enlargement and the
swelling thus produced may be suffi-
cient to close completely the respir-
atory passages. It is believed that
this high vascularity of the respira-
tory region is concerned with the
moistening and warming of the
current of respiratory air. The
secretions of this portion of the nose
are also believed to be inimical to
pathogenic germs and thus to afford
a protection to the deeper parts against the invasion
The olfactory region in man is yellowish in color as
it is in the calf and in the sheep. In the dog and the
rabbit it is of a more brownish hue. According to the
older anatomists it was supposed to extend in man over the
dorsal half or even more of the nasal cavity. Von Brunn
( 1892 ) , however, claimed by a reconstruction from sections
that the olfactory epithelium was much more restricted
than had been originally supposed. According to this
author only a small portion of the superior concha and a
correspondingly small part of the nasal septum represent
the unilateral area of distribution of the olfactory nerve.
This area in one subject measured 257 sq. mm. and in an-
Fio. 3. Respiratory epi-
thelium from the nasal cavity
of a young pig; b, basal cell;
c, ciliated cell. After Alcock,
1910, Fig. 2.
28 SMELL, TASTE, ALLIED SENSES
other 238 sq. mm. The more recent results of Bead
(1908), however, show that in man the olfactory fibers
spread from the dorsal portion of the olfactory cleft ven-
trally over the superior concha almost to its free edge and
correspondingly over the septum to about one third its ex-
tent. (Fig. 4). The antero-posterior spread of the nerve,
according to this author, is about twice
that of its ventral distribution on
either the concha or the septum;
hence the whole area innervated by
each olfactory nerve, if spread out
flat, would be approximately square
in outline and not far from 25 mm.
FIO. 4. olfactory deft to a side, somewhat over twice the
of man opened by turning MI -AT -r>
the nasai septum (s) up- extent ascribed to it by von Bruiin.
ward; the blackened area o /Mi? TI ' J.-L. T rrn ii>
shows the distribution of 6. Olfactory iLpithelium. The olfac-
the olfactory nerve. After "
Fig ad 3i 1908 ' Plate "' tory epithelium has been an object of
interest to histologists for a long
time. As early as 1855 Eckhard stated that in the frog
it was composed of two classes of elements, long epithelial
cells and nucleated fibers. Which of these served as the
endings of the olfactory nerve he was unable to say. In
the same year Ecker discovered on the deep face of the
olfactory epithelium a third class of cells subsequently
called by Krause (1876) basal cells. (Fig. 5.) These
three classes of elements were identified in a number of
vertebrates and described by Schultze (1856, 1862) who
expressed the belief that the nucleated fibers were sense
cells and represented the true endings of the olfactory
nerve though he was unable to demonstrate a connection
between these cells and the nerve.
It is probable that the connection of the so-called
NATURE OF SENSE ORGANS
olfactory cell with the olfactory nerve-fiber was first seen
by Babuchin in 1872 who showed that in a gold-chloride
preparation, fibers could be traced from the nerve to the
cells that were suspected by Schultze to be sense cells.
In 1886 this connection was demonstrated with much
greater certainty in methylen-blue preparations by
Ehrlich whose results were con-
firmed the following year
Because of the transitoriness
of methylen-blue preparations,
the results of Ehrlich and of
Arnstein were looked on with
some suspicion till they were
reproduced in Golgi preparations
by a number of workers, such as
Grassi and Castronovo (1889)
on the dog, Ramon y Cajal (1890)
on mammal embryos, Van
Gehuchten (1890) on rabbits, von
Brunn (1892) on man, Retzius
(1892a, 1892b, 1894) on fishes,
amphibians, reptiles, and mam-
mals, and by many other later
workers on various vertebrates.
( Fig. 6 ) . The results of all recent
students in this field of histology
support the statement that the
olfactory epithelium of vertebrates is composed of at
least three classes of cells : basal cells, ordinary epithelial
or substentacular cells, and sense cells from which the
olfactory fibers take their origin. Thus the opinion of
Fio. 5. Olfactory epith-
elium from a pig embryo
6H inches long; b, basal
cell; 0, olfactory cell; s, sus-
tentacular cell. After
Alcock, 1910, Fig. 10.
SMELL, TASTE, ALLIED SENSES
Sclmltze on this subject has been unquestionably and
The sustentacular cells are the chief supporting ele-
ments of the olfactory epithelium. Each of these cells
has a distal cylindrical portion that contains the yellowish
or light brownish pigment so characteristic of the olfac-
Fio. 6. Olfactory epithelium of a young mouse showing the olfactory cells and, to the right,
two suatentacular cells. Golgi preparation. After Retzius, 1892a, Plate 10, Fig. 2.
tory region. The nuclei of these cells are oval and con-
stitute the outermost zone of nuclei in the epithelium.
Their proximal portions are more or less irregularly
compressed and branched, hence the outlines of these
parts are commonly jagged.
The basal cells form a single row of block-like elements
on the proximal face of the olfactory epithelium. Their
short branching processes extend distally among the other
cells of the epithelium.
The olfactory cells are the most numerous of the three
classes of cells in the epithelium. Their nuclei are roundish
with well marked nucleoli and form the extensive nucle-
ated band between the distal zone of sustentacular nuclei
ANATOMY OF THE OLFACTORY ORGAN
and the less distinct proximal zone of basal nuclei. Each
of the olfactory nuclei is lodged in an oval cell-body.
Proximally this tapers rapidly into a fine olfactory nerve-
fiber which eventually enters the olfactory bulb of the
brain. Distally the body of the cell extends as a somewhat
coarser rod-like structure to the outer surface of the olfac-
tory epithelium where it ter-
minates in a small enlargement.
This enlargement has been
called the olfactory vesicle by
Van der Stricht (1909) who
ascribed to it a centrosomal
origin and believed it to play
a significant part in olfactory
reception. The olfactory vesi-
cle carries a cluster of proto-
plasmic filaments, the olfactory
hairs. (Fig. 7). These hairs
are apparently extremely deli-
cate and are easily destroyed;
hence they have escaped obser-j
vation by many workers. They were probably seen in the
frog as early as 1855 by Eckhard, but they were first gen-
erally identified and thoroughly studied by Schultze (1856,
1862) in a number of vertebrates. Apparently they are
never very numerous; Schultze (1862) found that in the
frog there were five to six hairs on each olfactory cell
(Fig. 8), and von Brunn (1892) and Kallius (1905) re-
corded six to eight in man. Eetzius (1894) noted two to
five hairs on each cell in the snake Tropidonotus. Ballo-
witz (1904) found ten to twelve or more in Petromyzon,
and Alcock (1910) states that in the pig the number varies
Fia. 7. Isolated olfactory cells
and sustentacular cells from man.
After von Brunn, 1892, Plate 30,
SMELL, TASTE, ALLIED SENSES
from five to eight, Because of their great delicacy the ol-
factory hairs are probably seldom observed to their full
length. Schultze (1856) described those of the frog as
long, but Jagodowski (1901) has shown
that in the pike the hair may be twice as
long as the olfactory cell itself, (Fig. 9)
and may reach from the distal end of the
cell through the whole thickness of the
superimposed slime. So delicate are the
distal portions of these hairs that
Jagodowski has proposed for them the
name of olfactory flagella or lashes. In
the opinion of this author the so-called
olfactory hairs are only the proximal ends
of these lashes, the distal part having
disappeared in the course of preparation.
The lashes can be demonstrated by means
of the Golgi method or by osmic acid.
These lashes are without doubt the true
receptive elements of the olfactory cells.
The secretion in which they are suspended
and whose thickness they probably
penetrate is produced by the numerous
olfactory or Bowman glands whose ducts
open out abundantly through the olfac-
4. Intermediate Zone. In the majority
of vertebrates there seems to be a fairly sharp boundary
between the respiratory epithelium and the olfactory
epithelium. In some mammals, however, these two regions
are separated by a considerable intervening area, known
as the intermediate zone. This was first described by
Fio. 8. Isolated
olfactory cell and
from a frog. After
Plate 1, Fig. 4.
ANATOMY OF THE OLFACTORY ORGAN 33
Grassi and Castronovo (1889) in the dog, and subsequently
was identified by Alcock (1910) in the pig. In this mammal
the epithelium of the intermediate zone is thicker than that
of the respiratory region and thinner than that of the
olfactory region. Besides basal cells it possesses two
types of epithelial cells, ciliated cells like
those of the respiratory epithelium and 1
non-ciliated sustentacular cells like those
of the olfactory region. It also contains
many olfactory cells, but these cells are
not as numerous in the intermediate zone
as they are in the olfactory region
where they are said to make up about
seventy per cent of the cells present. It
is plain from the accounts given that
the intermediate zone is a region of
transition between the two chief nasal FIQ.Q
preparation of an
regions, the olfactory and the respiratory. a lf pike ry (E e sox) ro in
5. Polymorphic Cells. In most verte- Srb.3? ffl
brates the olfactory cells exhibit great process ne arl"
. ,, . . ,, -.- ., , shown, but also the
umtormity 01 structure. In the fishes, ion g peripheral oi-
however, Dogiel (1887) has called attention
to a polymorphism among these elements,
and he has described in addition to the ordinary type of
spindle-shaped olfactory cell, cylindrical olfactory
cells and conical olfactory cells. These three types
have been identified by Morrill (1898) and by Asai
(1913) in a selachian (Mustelus) and by Jagodowski
(1901) in the pike (Esox). To what extent this
polymorphism occurs in other vertebrates and how
important it is for a right understanding of the action
of the olfactory organ has not yet been determined.
34 SMELL, TASTE, ALLIED SENSES
6. Sense Buds. In 1884 Blaue described what he be-
lieved to be sense buds in the olfactory epithelium of
certain fishes and amphibians. This observation was not
confirmed by later workers and it appears, as Betzius
(1892b) has remarked, that the so-called sense buds are
not true buds but folds or bands of olfactory epithelium
seen in transverse section. The buds subsequently de-
scribed by Disse (1896b) in the nose of the calf and shown
by him to be supplied by free-nerve terminations are be-
lieved by this author to be concerned with taste rather
than with smell. These structures, however, are claimed
by Kamon (1904) not to be true buds but bud-like
appearances produced by the mouths of the Bowman
glands. If this is so, no sense buds of any kind are
known in the olfactory epithelium of vertebrates.
7. Free-nerve Endings. In 1889 Grassi and Castronovo
with some uncertainty described from the epithelium
of the intermediate zone of the dog what they regarded as
free-nerve endings. Whether these were end-organs of
the olfactory nerve-fibers or not, they were unable to
determine. In 1892 similar endings were observed by von
Brunn at the border of the respiratory region in man.
Von Brunn believed these endings to be terminals of the
trigeminal nerve and, apparently by mistake, mentioned
Ramon y Cajal as their discoverer. Free-nerve endings
in the olfactory region were subsequently recorded by
Retzius (1892b) in the mouse and frog, by von Lenhossek
(1892) in the rabbit, by Morrill (1898) in Mustelus, by
Jagodowski (1901) in Esox, by Kallius (1905) in the
calf, and by Read (1908) in the kitten. Morrill 's obser-
vation for Mustelus has recently been confirmed by Asai
(1913). Hence there seems to be no doubt that in addition
ANATOMY OF THE OLFACTORY ORGAN 35
to the olfactory cells, free-nerve endings occur in the ol-
factory epithelium of vertebrates.
The source of the nerve-fibers from which the free-
endings of the olfactory epithelium arise is not definitely
settled. The fact that these endings may be very near the
outer surface of the olfactory epithelium shows that they
are not due to the incomplete impregnation of fibers from
the olfactory cells as was suggested by Van Gehuchten
(1890). Free-endings like those in the olfactory region
also occur in the respiratory region and here the only pos-
sible source for them is the trigeminal nerve; hence it is
probable that this nerve is also the source of the free-
nerve endings of the olfactory region. This opinion is sup-
ported by the observations of Rubaschkin (1903) who has
shown that in certain portions of the olfactory epithelium
of the developing chick the two sets of fibers, those from
the olfactory nerve and those from the trigeminal nerve,
take somewhat different courses and that the trigeminal
fibers are the fibers that give rise to the free-endings.
(Fig. 10). Thus such evidence as there is favors the
opinion first expressed by von Brunn and subsequently
reiterated by a number of investigators, that the free-
nerve endings of the olfactory region are from the tri-
geminal fibers. The vertebrate olfactory epithelium,
therefore, has two types of nerve terminations, olfactory
cells as the exclusive receptors for the olfactory nerve
and free-nerve endings as the probably exclusive endings
for the trigeminal nerve.
8. Development of Olfactory Nerve. Since the fibers
from the olfactory cells pass as olfactory nerve-fibers
to the olfactory bulb and terminate there without direct
connections with any other cells, the olfactory cells in the
SMELL, TASTE, ALLIED SENSES
nasal epithelium must be their cells of origin, as in fact
was shown to be the case for the chick by Disse (1896a,
1897). Here the olfactory nerve-fibers have been demon-
strated to grow from certain olfactory epithelial cells into
the olfactory bulb, the epithelial cells acting in all respects
like neuroblasts Bedford (1904). The trigeminal fibers
Fio. 10. Olfactory epithelium of an embryo chick (ninth day) showing olfactory cells,
sustentacular cells, and free-nerve endings of fibers from ganglion cells of the trigeminal
nerve. After Rubaschkin, 1903, Fig. 3.
must on the other hand grow from trigeminal ganglion
cells into the olfactory epithelium there to terminate as
free-nerve endings, but of this there is at present no di-
9. Comparative Anatomy of Olfactory Organs. The
nasal organs in the lower vertebrates are very different
from those in man. In Amphioxus a single sensory pit
slightly to the left of the median dorsal line of the head
and connected with the anterior end of the nerve-tube is
assumed to be an olfactory organ. If this is so, it is prob-
able that this pit corresponds to the single median olfac-
ANATOMY OF THE OLFACTORY ORGAN 37
tory sac in the cyclostomes notwithstanding the fact that
this sac shows evidence in its deeper parts of being a
double organ. In consequence of single nasal openings
Amphioxus and the cyclostomes are commonly contrasted
with other fishes, and in fact with all other vertebrates,
and are called monorhine. Those in which the olfactory
organs are obviously paired have been designated as
In the sharks and rays the paired olfactory pits are
situated usually on the ventral side of the snout. (Fig. 11) .
The single opening of each pit is more or less divided by a
fold of skin into an anterior inlet and a posterior outlet
the latter sometimes leading into the mouth. As the fish
swims through the water and particularly as it takes
water into its mouth in breathing, a current of water is
passed through eadi of its olfactory sacs. In this way the
olfactory organs become associated with the respiratory
current, a condition that is more pronounced in the lung-
fishes than in the sharks and rays, for in the lung-fishes
the anterior apertures are external and form true anterior
nares, and the posterior openings lie within the mouth
and correspond to the choanas of higher vertebrates. In
the highly specialized bony fishes, the paired olfactory pits
are almost always on the dorsal aspect of the head and
quite distant from the mouth. Each pit has two entirely
separate openings, an anterior inlet and a posterior outlet.
By means of these two openings a current of water enters
and leaves each pit. This current is produced either by
ciliary action within the pit ( Amiurus) or by the action of
the muscles associated with the jaws and gills (Fundulus).
In bony fishes, then, the olfactory pits are purely recep-
tive and are in no direct way connected with the respira-
SMELL, TASTE, ALLIED SENSES
tory current as they are in the sharks and rays, and in
In the air-inhabiting vertebrates each olfactory sac
possesses, as in man, an external inlet, the anterior naris,
and a posterior outlet, the choana, opening into the mouth
or the pharynx. The olfactory sacs are relatively simple
in amphibians, but become progressively more compli-
Fia. 11. Ventral view of the head of a shark (Scyllium) showing the olfactory pita in rela-
tion to the mouth.
cated in reptiles and birds, and vastly more so in mam-
mals. Here the surface of the sac is enormously extended
through the development of lateral folds or conchae which
may be further complicated by the production of second-
ary folds. In mammals the more ventral of these conchaa,
those attached to the maxillary bone, are apparently not
concerned with olfaction, but lie in the purely respiratory
region of the nasal chamber. The more dorsal conchae
those from the ethmoid bone, serve as olfactory surfaces.
It has been shown that in some mammals, as for instance
in Orycteropus, there may be upwards of ten olfactory
ANATOMY OF THE OLFACTORY ORGAN 39
conchae. Forms that possess these larger numbers of
conchas are known to be keen-scented and are termed mac-
ro smatic. Those in which the number of olfactory conchae
is small, four or fewer, such as the seals, some whales,
monkeys, and man, are known to be less acute of smell
and are called microsmatic. Others again, such as the
toothed whales, porpoises and the like, in which the olfac-
tory organ has almost completely degenerated, are sup-
posed to be devoid of olfaction and are called anosmatic.
In such forms the nasal cavities have lost their original
sensory function and have come to be of importance only
in connection with respiration, a purely secondary relation.
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40 SMELL, TASTE, ALLIED SENSES
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PHYSIOLOGY OF OLFACTION.
Contents. 1. Nerves of Olfaction. 2. Passage of Air
through the Nasal Cavity. 3. Minimum Stimulus.
4. Physical Condition of Stimulus, Gas or Solution? 5.
Olfaction in Fishes. 6. Fatigue and Exhaustion. 7. Quali-
ties of Odors. 8. Chemical Relations of Odors. 9. Inade-
quate and Adequate Stimuli. 10. Olfactory Reflexes.
1. NERVES of Olfaction. The olfactory region of verte-
brates has been shown to possess olfactory cells as ter-
minations of the olfactory nerve and free-nerve endings
representing in all probability the trigeminal nerve. It
has long been the opinion of investigators that the olfac-
tory sense is mediated by the endings of the olfactory
nerve, but this opinion has not been without its opponents.
Thus Magendie, in a series of publications beginning in
1824, came to the conclusion that the trigeminal nerve was
the nerve of olfaction and that the so-called olfactory
nerve was one whose function was wholly unknown. His
opinion was based in part upon experiments on the dog.
After the olfactory nerves of this animal had been cut, it
was found still to respond to acetic ether and to ammonia.
Even when blindfolded a dog with severed olfactory
nerves would seize cheese or meat but it would not eat
meat sprinkled with tobacco. It was pointed out by
Magendie 's critics that many of the stimulating sub-
stances used by him, such as ammonia and the like, not
PHYSIOLOGY OF OLFACTION 43
only possessed odor but were irritants for mucous sur-
faces generally and thus without reference to olfaction
could call forth vigorous responses. Magendie, however,
claimed that his results were not dependent upon these
substances, but could be demonstrated by the use of non-
irritants, such as lavender oil.
Magendie 's opinion that the trigeminal nerve was the
nerve of olfaction was opposed almost from the beginning.
Eschricht in 1825 pointed to numerous cases of persons
who were anosmic in consequence of the absence of the
olfactory nerve or of its degeneration. Bishop in 1833
described a case of paralysis of the trigeminal nerve in
which there was, however, full retention of olfaction.
Picht (1829) and Duges (1838), both of whom were incap-
able of olfaction in the ordinary sense of the word, were
nevertheless easily stimulated through their nasal mem-
branes by the vapor of acetic ether, or of ammonia. Val-
entin (1839) found that a normal rabbit would sniff the
body of a dead one, but that a rabbit whose olfactory
nerves had been cut would not thus respond. Schiff
(1859) experimented on five pups, in four of which the
olfactories were severed, the fifth being retained in a nor-
mal condition as a control. After recovery from the op-
eration, the four pups in which the nerves had been cut
were unable to find the mother's nipples, and did not dis-
tinguish between a man and the mother though they
turned their heads away and sneezed when ammonia or
ether was administered. Acetic acid stimulated them only
when its vapor was very concentrated. These and many
other similar results completely overthrew Magendie 's
contention and showed that, though the trigeminal
endings were concerned with the reception of what may be
44 SMELL, TASTE, ALLIED SENSES
called irritants, true olfaction was accomplished only
through the olfactory terminals, which have to do with
delicate perfumes, aromas, and the like, many of which
were associated with food.
The recognition in nasal stimulation of the two classes
of substances, irritants acting on trigeminal terminals,
and true odors affecting the olfactory endings, is of funda-
mental importance, and the failure to appreciate this
distinction is responsible in part at least for much of the
confusion that exists in what has been written on the
olfactory stimulus. As early as 1851 Frohlich pointed out
this distinction and called attention to the fact that irri-
tants or stimuli for the fifth nerve ordinarily induce
vigorous reflexes, respiratory and the like, whereas true
odors are in nature much milder and seldom call forth
strong responses. It is quite possible that some materials
are stimuli for both classes of end-organs; thus tobacco
smoke not only carries with it an aroma or true odor but
also acts as an irritant, These two actions, however, may
depend upon different chemical substances in the smoke.
Other stimuli such as oil of mustard or possibly ammonia,
that are chemically much more homogeneous than tobacco
smoke, may affect, nevertheless, both sets of receptors and
thus exhibit the characteristics of both irritants and
true odors. A revision of the so-called olfactory stimuli
from this standpoint is much to be desired.
2. Passage of Air through the Nasal Cavity. In ordi-
nary respiration in man the passage of air through the
nasal cavity does not necessarily excite olfaction at once.
Sooner or later, however the odor may be slightly sensed
after which a few deep breaths or sniffing movements
are usually made, whereupon full stimulation ensues.
PHYSIOLOGY OF OLFACTION 45
The course that the current of air takes through the
nasal chamber during quiet respiration has been studied in
several ways. Paulsen in 1882 published the results of ex-
periments 011 the human cadaver. He opened the nasal
cavity by sawing through the head of a cadaver close to
the median plane. Pieces of red litmus-paper were then
placed on different parts of the nasal surface and the two
halves of the head were brought together again. By
means of a bellows attached to the trachea of the cadaver,
the current of air that in life passes through the nasal
chambers was imitated. This artificial current was
charged with ammonia and thus a means was given of in-
dicating the spread of the current by the location of the
pieces of litmus-paper that changed from red to blue. As
a result of this test it was found that the inspired air took
a curved course from the naris to the choana. (Fig. 12).
The highest part of this curve was near the middle of the
nasal cavity, but this never reached a point as high
as the olfactory cleft. When the current was reversed
by causing it to enter at the choana and emerge at
the external naris, as in expiration, the direction of the
current was found to be much the same as in inspiration
except that a somewhat lower course was followed. Thus
in both inspiration and expiration the current of air is
limited to what is more generally regarded as the respira-
tory region of the nasal cavity, the olfactory region being
Paulsen 's results were confirmed in all essential par-
ticulars by a number of later investigators including
Franke, Zwaardemaker, Danziger, and Rethi, who worked
on dead animals and human cadavers by methods not un-
like those used by Paulsen. Franke (1893) sawed open
46 SMELL, TASTE, ALLIED SENSES
the head of a human cadaver in the median plane, replaced
the nasal septum with glass and by means of an artificially
produced respiratoiy current showed that smoke in its
passage through the nasal cavity remained in the so-called
respiratory region. He observed, however, that both in-
spiration and expiration were accompanied by strong
Fio. 12. Diagram of the right nasal cavity of man laid open and showing by arrows the
direction of the inspired current of air over the nasal septum (right half of figure) and over
the lateral wall (left half of figure). After Paulsen, 1882.
eddies in the moving air. Kayser (1890) aspirated very
light magnesia powder into the respiratory current of a
quietly breathing normal subject and then inspected the
nasal surfaces by means of a rhinoscope. The magnesia
particles accumulated on the moist surfaces of the respir-
atory portions of the nose and not on those of the olfac-
tory region, thus confirming Paulsen 's results but by a
method that was by no means so artificial as that employed
by other workers. It may, therefore, 'be regarded as
PHYSIOLOGY OF OLFACTION 47
fairly well established that the current of air that sweeps
through the nasal cavity in quiet respiration is limited
chiefly to the non-olfactory portion of that cavity. Ac-
cording to Paulsen and to Zwaardemaker this current
even in its eddying effect does not rise above the lower
edge of the middle concha or at most, according to
Franke, the lower edge of the superior concha. This
limitation is probably more pronounced in expiration
than in inspiration.
Although the experimental evidence does not show that
the respiratory current spreads to the olfactory surface
of the nose, odorous particles must in some way reach this
situation. Zwaardemaker (1895) was led to believe that
the diffusion of these particles played an important part
in this process, but diffusion is a relatively slow operation
and it is very doubtful if it is a significant factor in carry-
ing the odorous material to the olfactory receptor. It
seems more probable that the shifting pressures that
accompany respiration and the slight eddies that are
formed in the general current are responsible for a grad-
ual change of air in the olfactory cleft. The change thus
produced is probably too slight to be detected easily by
the means heretofore employed in tracing the current and
yet it may be sufficient to initiate such olfaction as occurs
in quiet respiration. Olfaction thus once begun would
naturally excite sniffing and this process seems to be
entirely sufficient to account for a rapid change of air in
the olfactory cleft whereby olfaction would be brought to
full height. Thus air currents are certainly the chief if
not the sole factors concerned with transporting the
odorous particles to the olfactory membranes.
The accumulation of odorous materials on the olfac-
48 SMELL, TASTE, ALLIED SENSES
tory surfaces may be much intensified by the condensa-
tion of moisture within the nasal cavity. Zwaardemaker
(1917) has called attention to the fact that a fog formed
from a vaporized salt solution is very much less stable
when it includes odorous substances than when it does
not. This condition is believed to depend upon the elec-
tric charges earned by the particles concerned, and
Durand (1918a, 1918b) recently claimed that olf action
is more or less dependent upon an appropriate hygro-
metric state in the olfactory atmosphere and that what-
ever facilitates the condensation of watery vapor there
Among the older physiologists Bidder (1844) main-
tained that olfaction was possible on inspiration and that
expired air could not stimulate the organ of smell. Paul-
sen's observations show that this opinion is improbable
and the direct test of breathing odorous air in through
the mouth and out through the nasal cavity has de-
monstrated that it is quite erroneous. The olfactory
sensations produced on expiration are noticea.bly less
than on inspiration and this is probably due partly to the
lower course maintained in the nasal cavity by the ex-
pired air and partly to the previous elimination of much
of the odorous material by attachment to the moist sur-
faces of the mouth, pharynx, and other parts over which
the air passes on its way to the nasal chamber. Never-
theless, as Nagel (1904) has pointed out, the odors of our
food during mastication are the results of stimulating
material that reaches the olfactory surfaces through the
choanaB rather than through the external nares. The
importance of these odors in promoting the various kinds
PHYSIOLOGY OF OLFACTION 49
of digestive reflexes, muscular, secretory, and so forth,
has long been recognized.
3. Minimum Stimulus. The common belief that the
olfactory stimulus consists of minute material particles
suspended in the air current of the olfactory organ is
supported by the observation that odors may be carried
on the wind in a definite direction many miles. Odors
do not emanate from a given center and disperse in all
directions as sound and light do. Moreover many sub-
stances, such as arsenic, that are odorless under ordinary
circumstances, give out an odor after they have been
heated sufficiently to volatilize. The fact, discovered in
1917 by Woodrow and Karpman, that the adaptation time
for olfaction the time needed for an olfactory sensation
to wane completely is directly proportional to the vapor
tension of the odorous material shows that olfactory
stimulation is due to the activity of gaseous particles.
These and other like observations have led to the conclu-
sion, now generally accepted, that the olfactory organs
are normally stimulated by material particles, and not
by disturbances of a non-material character.
Some odorous bodies such as musk are well known
to give out these material particles for a very considerable
time without appreciably changing weight. From the
standpoint of the receptor this indicates that olfaction
is called forth by an infinite simally small amount of sub-
stance, and measurements directed toward testing this
question justify the conclusion. These measurements
have been made in a variety of ways.
One method of procedure is that of evaporating a
given weight of odorous material in a known volume of air
and then testing the air by sniffing it. This method lends
50 SMELL, TASTE, ALLIED SENSES
itself readily to the determination of absolute measure-
ments but it is not so easily applied to questions involv-
ing the comparison of odors. For the measurement of
olfactory acuity, but especially for the comparison of
odors, Zwaardemaker invented an ingenious piece of ap-
paratus called an olfactometer. (Fig. 13). This consists
of two tubes that slide one within the other and so shaped
that one end of the inner tube may be applied to the
nostril. The odorous material is carried on the inner
surface of the outer tube. When the inner tube, which
is graduated, is slipped into the outer one so as to cover
i i i i i i i i r
FIG. 13. Simple rubber olfactometer. After Zwaardemaker, 1895, Fig. 14.
completely its inner face and air is drawn, into the nostril
through the tube, the odorous surface being covered gives
out no particles and no odor is perceived. If, now, the
inner tube is withdrawn a certain distance so that a
given surface of odorous material is exposed to the cur-
rent of air, odorous particles escape into the current and
these may be sufficient in amount to call forth olfaction.
By adjusting the inner tube in relation to the outer one
whereby more or less of the odorous surface is exposed,
a point can be found where minimum stimulation occurs.
The amount of odorous substance delivered under these
circumstances to the air current has been designated by
Zwaardemaker as an olfactie, the unit of olfactory stimu-
lation. Having determined for a given substance the
area necessary for the delivery of one olfactie, doubling
PHYSIOLOGY OF OLFACTION
that surface by an appropriate movement of the inner
tube will produce a stimulus of two olfacties and so
forth. Thus a graded series of measured olfactory stim-
uli can easily be obtained. Further, by using outer
tubes carrying different odorous substances, various com-
parisons can be instituted as measured in olfacties.
Moreover, a double olfactometer (Fig. 14) may be easily
14. Double olfaotometer. After Zwaardemaker, 1895, Fig. 15.
devised in that two single olf actometers may be combined
so that one current carrying an odorous material of a
given concentration may be introduced into one nostril
and another carrying a second odorous substance of
known concentration can be introduced into the other
nostril, or both currents may be united and the odorous
mixture thus produced can be let into one nostril. Thus
a variety of comparisons may be easily made.
Van Dam (1917b) has modified Zwaardemaker 's ap-
paratus by applying the odorous material in the form of
52 SMELL, TASTE, ALLIED SENSES
a rod instead of a coating to the inside of a tube. The
rod is made of paraffin mixed in a definite proportion with
the odorous substance and the extent to which the rod is
exposed in the olfactometer tube is a measure of the
concentration of the odorous particles in the air current.
Rods of metal, platinum, gold, or zinc, have also been
used ; these have been charged by immersing them in an
atmosphere of odorous material for a given length of
time and then tested. The odorous particles gather on
their surfaces and are subsequently freed. The success of
this method makes it clear tha.t in the original evapora-
tion method more or less of the odorous material must
become ineffective in that it adheres to the walls of the
container in which the evaporation is carried out.
As a means of avoiding these and other difficulties
Allison and Katz (1919) have recently employed in the
testing of stenches a type of odiometer that for accuracy
of work bids fair to replace most of the other devices.
It consists of a number of Venturi-type flow-meters so
arranged that a measured volume of air can be passed
at a uniform rate through or over the chemical, and this
air is then mixed with another measured volume of pure
air also flowing at a uniform rate. The concentration of
the chemical is measured by determining its loss in
weight after a measured volume of air has passed
through or over it. From this loss of weight and the to-
tal volume of air with which the chemical has been mixed,
the concentration in milligrams per liter of air is deter-
mined. The mixture of air and chemical passes finally
through a tube with a glass funnel at the open end. The
funnel is placed over the nose of the person who by a sin-
gle inhalation tests the mixture. The odors are rated ar-
bitrarily as detectable, faint, quite noticeable, strong or
PHYSIOLOGY OF OLFACTION 53
very strong. By this means extremely accurate quanti-
tative results can be obtained.
In testing olfactory acuity the majority of workers
have used the method of evaporating a known weight of
substance in a given volume of air. By this method Val-
entin (1848) found that 1/2,000,000 of a milligram of oil
of rose per cubic centimeter of air was odorous. Assum-
ing that 100 cubic centimeters of this mixture were nec-
essary for olf action, he concluded that the total weight of
oil of rose used in this operation was the very small
amount of 1/20,000 of a milligram. Valentin also found
that water containing 1/2,000,000,000 of its weight of
tincture of musk had a perceptible odor whereas water
containing only 1/3,300,000,000 of this tincture could not
be distinguished from ordinary water. One gram of the
odorous mixture called forth the characteristic smell and
contained only 1/2,000,000 of a milligram of tincture
More significant measurements were made by Fischer
and Penzoldt (1886) on chlorphenol and mercaptan. One
milligram of chlorphenol was evaporated in a room of
230 cubic meters capacity and was thoroughly mixed with
the air. This dilution called forth an unquestionable ol-
factory sensation. It contained 1/230,000,000 of a milli-
gram of chlorphenol per cubic centimeter of air or, if it
is assumed that 50 cubic centimeters of air are the mini-
mum needful for olfaction, the total amount of chlor-
phenol necessary was found to be 1/4,600,000 of a
milligram. By a similar method it was shown that
mercaptan, a liquid with a penetrating garlic odor, could
be recognized at a concentration of 1/23,000,000,000 of a
milligram per cubic centimeter, a concentration that
would yield 1/460,000,000 of a milligram for every 50
54 SMELL, TASTE, ALLIED SENSES
cubic centimeters of air. Notwithstanding this infinites-
imally small amount of mercaptan, the quantity, just
designated was estimated by von Frey (1904) to contain
some 200,000,000,000 molecules of mercaptan.
Passy (1892a, 1892b) has made similar minimum de-
terminations for a number of substances and has shown
that artificial musk, probably the most powerful of all
known odorous materials, is about a thousand times
stronger than natural musk. In his other determinations
he found that olfactory acuity ranged in thousandths of
a milligram per liter of air from camphor at 5 to vanillin
at from 0.005 to 0.0005. The last determination may be
expressed as equivalent to 1/2,000,000,000 of a milligram
of vanillin in a cubic centimeter of air, a high dilution
but still not so extreme as that already recorded by
Fischer and Penzoldt for mercaptan.
The details of the more important of Passy 's deter-
minations are given in the following table in which ol-
factory acuity, as measured by the minimum amount of
substance that was stimulating to the several persons
tested, is expressed in thousandths of a milligram per
liter of air.
Minimum concentrations for olfaction in thousandths of a milligram of
suhstance per liter of air (Passy, 1892b).
Substances Thousandths of a milligram
Citral 0.5 to 0.1
Heliotropin 0.1 to O.u.">
Cumarin 0.05 to 0.01
Vanillin 0.005 to 0.0005
Passy (1892c) has also determined the minimum con-
PHYSIOLOGY OF OLFACTION 55
centration necessary for the olfaction of a number of
alcohols. These determinations have been recorded in
millionths of a gram per liter of air and are given
in Table II.
Passy 's determinations indicate that the lower alco-
hols have relatively faint odors, but that the higher
Minimum concentrations for olfaction, in millionths of a gram of alcohol
per liter of air (Passy, 1892e).
Alcohol Primary Secondary Tertiary
Propyl 10 to 5 40
Normal Butyl 1 20 to 10
Normal Amyl 40 to 20
Active sinistral Amyl 0.6
Inactive Isoamyl 0.1
members of the series are fairly comparable with, for
instance, the essential oils. A determination for ethyl
alcohol by Parker and Stabler in 1913 showed that this
alcohol could be detected only to a concentration of about
5.75 milligrams of alcohol per liter of air. The smaller
amount found by Passy, namely 0.25 milligrams per liter
of air, is believed by these authors to be due to odorous
impurities that were found by them in certain ethyl
alcohols and that may have been present in those tested
Some of the more striking determinations by Allison
and Katz (1919) are reproduced in Table III.
Here it will be noted that the most active mercaptan
tested, propyl mercaptan, is detectable at a concentra-
SMELL, TASTE, ALLIED SENSES
tion of 0.006 milligrams per liter of air which is equal
to 6/1,000,000 of a milligram per cubic centimeter.
This determination is by no means so extreme as that
of Fischer and Penzoldt, 1/23,000,000,000 of a milligram
per cubic centimeter. Whether this difference is due to a
difference in the compounds used, for Fischer and
Concentrations in Milligrams of Chemical per liter of air.
Intensity of Odor
Ethyl mercaptan . .
Oil of peppermint
Methyl isothiocyanate . . .
Penzoldt do not state what mercaptan they tested, or
whether it represents a difference in the methods em-
ployed cannot be stated. In the table from the work of
Allison and Katz, as in all previous sets of determination,
artificial musk is shown to be without question the most
stimulating substance tested and thus stands at the head
of olfactory stimuli.
Notwithstanding the numerous discrepancies between
the various sets of determinations for olfactory acuity
made by various workers, it must be admitted that olfac-
PHYSIOLOGY OF OLFACTION 57
tion is accomplished through very small, often infinitesi-
mally small, amounts of material, and yet these amounts
involve immensely large numbers of molecules of the
4. Physical Condition of Stimulus, Gas or Solution!
In olfaction in the air-inhabiting vertebrates the stimu-
lus has been generally assumed to be material particles
in a vaporous or gaseous condition and not, for instance,
in the form of a solution.
This opinion was long ago supported by the experi-
ments of Tourtual (1827) and especially of Weber (1847)
both of whom believed that it could be shown that sub-
stances that could be smelled as vapors could not be
smelled as solutions when introduced as such into the nose.
Thus Weber was unable to recognize cologne water when
this liquid, much diluted with ordinary water, was poured
into his nasal cavities. He, therefore, concluded that
though the vapor from cologne water was easily smelled,
a solution of it was not so sensed and that hence the
vaporous state of the substance was necessary as a stim-
ulus for the olfactory organ. This conclusion was ac-
cepted by a number of investigators including Nagel
(1894, 1904), Zwaardemaker (1895), Haycraft (1900)
Aronsohn, in 1884, pointed out the great influence
that water and temperature had on the olfactory organ.
Ordinary cold water when introduced into the nose will
so affect the organ of smell that olfaction is impossible
for some time to come. Cold water is known to excite
an increased production of mucous whose volume would
materially interfere with stimulation by covering up the
olfactory surfaces. Moreover if the action of water on
58 SMELL, TASTE, ALLIED SENSES
the organ of smell in an air-inhabiting vertebrate is
continued for some time, it is said to result ultimately
in the destruction of the olfactory hairs. Thus Schultze
(1862) noted that when the olfactory membrane of an air-
inhabiting amphibian is flooded with water, the cilia with
which it is provided may continue to beat for hours,
but the much longer and heavier olfactory hairs vanish
almost at once. To minimize this deleterious effect Aron-
sohn, therefore, introduced into the nose material dis-
solved, not in ordinary water, but in physiological salt
solution and at an appropriate temperature. With these
precautions he claimed that it was very easy to recognize
weak solutions of clove oil. Vaschide in 1901 confirmed
Aronsohn's results and pointed out that temperature
was a more important factor in carrying out conclusive
tests than the composition of the solvent.
These results, which were in direct opposition to those
of Weber, were criticized by Zwaardemaker (1895) and
especially by Veress (1903) who showed that the pro-
cedure employed by Aronsohn probably resulted in a
failure to fill the olfactory cleft. Veress maintained that
unless great care was taken at this step, air was very
likely to remain in this cleft and thus the solution that
was being tested would never really reach the olfactory
terminals. Under such circumstances odorous particles
would escape from the solution into the air filling the cleft
and thus reach the olfactory organ as in ordinary olfac-
tion. Thus it became necessary in making a conclusive
test to take steps to insure the complete filling of the
olfactory cleft with the solution to be tested. After
some experimentation on the human cadaver, Veress per-
fected a technique whereby this could be accomplished.
PHYSIOLOGY OF OLFACTION 59
On thus introducing odorous solutions into the nasal
chambers of a living subject, he found that these solutions
were stimuli for the olfactory organs, but that they did
not produce the sensation ordinarily associated with
them. A person, however, could soon learn to associate
a given sensation with a particular substance and could
thus acquire an ability to recognize this substance, but not
by what would be called its proper odor. Veress, there-
fore, concluded that though solutions of odorous materials
are stimuli for the olfactory organs, they are inadequate
rather than adequate stimuli. It thus appears, contrary
to the results obtained by Weber, that the olfactory
organs of an air-inhabiting vertebrate can be stimulated
by ordinary solutions, though this form of stimulation
cannot be looked upon as normal.
To deny that the olfactory organs of man and other
like vertebrates are stimulated by solutions, as has been
done by a number of workers, implies a certain lack of
appreciation of the actual environment of the olfactory
terminals. These are the olfactory hairs that project in-
to the coating of mucous that covers the olfactory mem-
brane. These hairs appear to be completely covered by
the mucus and should any of their lash-like ends reach to
the outer surface of this layer, they are certainly far too
delicate to project into the adjacent air; they would
unquestionably remain within the limits of the mucous
layer. Thus the olfactory hairs are at all times sur-
rounded by watery mucous, which is in contact on its outer
face with the air carrying the odorous particles. These
particles, as already indicated, must be caught in great
numbers on the moist mucous surface, absorbed according
to Zwaardeniaker (1918b), and, since they are in the form
60 SMELL, TASTE, ALLIED SENSES
of gaseous or vaporous particles, they probably enter
quickly into solution in the watery mucous and in this state
come in contact with the olfactory hairs. From the nature
of the surroundings, then, it would seem extremely im-
probable that the stimulating material for the olfactory
terminals should be in any other state than that of a solu-
tion. This opinion seems to be gaining ground rapidly
among the more recent workers, for it has found clear
expression within the last few years in papers by Back-
man (1917a), by Durand (1918b), and in a qualified way
by Henning (1916).
As already indicated, the difficulty met with in at-
tempting to stimulate adequately the human olfactory
epithelium with solutions of odorous material is due in
all probability to the effects of the solvent on the olfac-
tory hairs and not to the incapacity of these terminals to
be stimulated by solutions. These hairs are apparently
very delicately attuned to a mucous environment that
would be very difficult to duplicate experimentally and yet
this environment seems to be essential to a wholly suc-
cessful test. Care as to temperature and salt contents of
the solvent, as emphasized by Aronsohn, Vaschide, and
Veress, are probably only the first steps in this direction.
The relation of the solubility of a substance to its
efficiency as an olfactory stimulus has been discussed
recently by Backman (1917a). This investigator has
expressed the opinion that not only the aqueous environ-
ment of the olfactory hairs must be considered but also
the substance of the hairs themselves. This he believes
to be lipoid in character, an opinion that is supported by
the well known fact that these hairs are best demon-
strated by osmic acid. If the embedding mucous layer is
PHYSIOLOGY OF OLFACTION 61
watery and the olfactory hairs oily, it follows that any
substance that gains entrance into the body of the hair
must first have been dissolved in water and then in oil.
From this standpoint Backman attempted to determine
whether there was any relation between the effectiveness
of certain olfactory stimuli and their solubility in water
and in oil. Water and olive oil, each at 30 degrees centi-
grade, were used as the test solvents. Thus methyl alco-
hol and ethyl alcohol, which are without strong odor,
were found to be freely soluble in water, but only very
slightly soluble in oil. Hence while they would dissolve
abundantly in the olfactory mucous, they would fail to
enter the hairs to any great extent in consequence of which
their effectiveness as stimuli must be, according to Back-
man, very slight. On the other hand normal butyl alco-
hol has a strong odor and its efficiency as a stimulus was
believed to depend upon the fact that it is soluble in water
to the extent of 8.3 per cent and in oil to an almost indefi-
nite amount. Other substances showed somewhat different
relations. Thus chlorbenzol could be detected at a dilu-
tion of 6.7 x 10."* gram-molecules per liter of air, and is
soluble in water to the extent of 0.25 per cent and in oil
indefinitely. Brombenzol could be smelled at the some-
what greater dilution of 1.1 x 10.~ 8 gram-molecules per
liter of air; yet it is less soluble in water (0.045 per cent)
than chlorbenzol though indefinitely soluble in oil. In
these instances the degrees of solubility in water are the
reverse of the effectiveness of these two substances as
olfactory stimuli. Possibly solubility in oil, as intimated
by Larguier des Bancels (1912), is of much more signifi-
cance for olfactory stimulation than solubility in water.
If the olfactory hairs in man are provided with flagella,
62 SMELL, TASTE, ALLIED SENSES
such as have been described by Jagodowski (1901) in the
pike, and the distal ends of these flagella reach through
the olfactory mucous to the nasal atmosphere, the odor-
ous particles may come directly in contact with them
and dissolve in their lipoid substance without pass-
ing through an intermediate watery layer. In that case
solubility in lipoid would be the only form of solubility
necessary for the introduction of an effective stimulus.
That a number of odorous substances are more soluble
in lipoid than in water has recently been shown by Kremer
(1917) who found that larger quantities of citral.
guaiacol, pyridine, and even chloroform and ether would
dissolve in a saturated aqueous solution of lecithin than
in pure water. Of course the varying capacity for re-
action of such materials as may thus become dissolved
in the substance of the hairs must profoundly influence
stimulation and possibly it is in this direction that the
difference between such substances as chlorbenzol and
brombenzol is to be explained. But however these de-
tails may be worked out eventually, the general opinion
that olfactory stimulation is dependent upon some form
of solution seems to be beyond question.
That the material thus dissolved must act chemically
on the olfactory receptors and not by means of any radia-
tion that it may give out seems probable from the fact
that olfactory stimuli are substances that are not known
to be radio-active. That there is a kind of physiological
radio-activity, such as has been claimed recently for po-
tassium by Zwaardemaker (1918a, 1920) and as might be
urged for the materials of olfactory stimulation, seems
extremely improbable from the recent work of R. F. Loeb
(1920) and of J. Loeb (1920). Moreover it would be
PHYSIOLOGY OF OLFACTION 63
very difficult to explain the variety of olfactory sensa-
tions on the basis of stimulation by radio-activity, but
the assumption that the stimuating materials act chemi-
cally on the substance of the receptor is in easy accord
with the diversity of olfactory experience.
5. Olfaction in Fishes. It has already been pointed
out that most fishes possess paired olfactory sacs whose
structure and innervation are essentially indentical with
the corresponding parts in the air-inhabiting vertebrates.
Nevertheless currents of water flow through these sacs
and such stimulation as they receive must come from
these currents. Nagel (1894), who was one of the most
vigorous opponents to the idea that the olfactory organs
were stimulated by solutions and believed that gases or
vapors were the only real stimuli for these receptors,
was led to conclude that the so-called olfactory organs of
fishes were fundamentally different from those of the
air-inhabiting forms and that they probably more nearly
resembled organs of taste than any other receptor pos-
sessed by the higher animals. This opinion was based
upon theoretic considerations rather than upon any par-
ticular observation or test.
But before these views had been expressed by Nagel,
a certain amount of experimental evidence concerning
olfaction in fishes had been gathered. This was prelimi-
nary in character and inconclusive, but it nevertheless
paved the way for further advance. Thus the observa-
tion of Aronsohn (1884a), that goldfish, which ordinarily
will eat ant pupae with avidity, would not take these pupae
after they had been smeared with a little oil of cloves,
is not final evidence that the fish scented the oil, for
it is entirely possible that this oil irritated the skin of the
64 SMELL, TASTE, ALLIED SENSES
fish's snout and did not stimulate the olfactory apparatus
at all. Nor was the discoveiy made by Steiner (1888),
that the spontaneous appropriation of food by the shark,
Scyllium, ceases on the removal of the cerebral lobes or
simply on cutting the connection between these lobes and
the olfactory bulbs, satisfactory evidence that the olfac-
tory apparatus in these fishes is an organ of smell rather
than a receptor for taste or some closely allied sense.
Nagel (1894) noted that the front of the head of the fish,
Barbus, was as sensitive to sapid substances after the
olfactory tracts had been cut as before that operation, and
Sheldon (1909), who studied the dogfish with great ful-
ness, demonstrated that the decided sensitiveness of the
nostrils of this fish to weak solutions of oil of cloves,
pennyroyal, thyme, and the like, was not influenced by
severing the olfactory crura, but disappeared on cutting
the combined maxillary and mandibular branches of the
trigeminal nerve. Evidently the nasal surfaces of fishes
like those of the higher vertebrates, are innervated by
fibers from the trigeminal nerve, and it is this nervous
mechanism rather than the true olfactory apparatus, that
is stimulated by the substances that have ordinarily been
applied by experimenters. In 1909, Baglioni showed that
blinded fishes were excited by the presence of food. None
of these experiments, however, demonstrated conclu-
sively that smell rather than taste or some other allied
sense, was concerned as the receptor.
As early as 1895 von Uexkull observed that dogfishes
from which the olfactory membranes had been removed
did not respond to the presence of food whereas normal
dogfishes three to five minutes after food had been in-
troduced into their tank, sought it with great eagerness.
PHYSIOLOGY OF OLFACTION 65
In these experiments no attempts were made to exclude
sight or to ascertain the effects of the operation. In ex-
periments carried out by me in 1910 an attempt was
made to gain more conclusive evidence. Five normal
catfishes (Amiurus) were allowed to swim in an aqua-
rium in which were hung two wads of cheese cloth one
containing concealed earthworms, and the other made
of cloth only. In the course of an hour the wad con-
taining the worms was seized eleven times by the fishes
notwithstanding the fact that from time to time this wad
was interchanged in position with the other. During the
same period the wad without worms was passed over by
the fishes many times and never excited any noticeable
Ten catfishes were next prepared for further experi-
mentation; in five of these the olfactory tracts were cut
and from the remaining five the barbels, the seat of the
chief external gustatory organs, were removed. After
the fishes had recovered from these operations, they were
put in an aquarium into which was introduced a wad of
cheescloth containing minced earthworms. During the
first hour the wad was seized 34 times by fishes without
barbels but with normal olfactory organs and, though
often passed over by fishes with cut olfactory tracts, it
was never seized by any of these and " nosed" only once
by one of them. None of these fishes paid any attention
to a wad of cloth containing no worms. Repetitions of
these tests gave uniformly similar results and led to the
conclusion that the olfactory organs of the catfish are
serviceable in sensing food at a distance much beyond
that at which the organs of taste are capable of acting;
in other words, catfishes truly scent their food.
66 SMELL, TASTE, ALLIED SENSES
Similar experiments on the killifish (Fundulus) gave
like results Parker (1911). Here, however, the olfac-
tory organs were excluded, not by cutting the olfactory
tracts, but by stitching up the anterior nares. As a re-
sult of this operation the fish no longer responded to hid-
den food, but quickly reacquired this power after the
Fia. 15. Ventral view of the head of a Hammer-head Shark (Cestracion) showing the
olfactory pita (o) widely separated. After Carman, 1913, Plate 1, Fig. 2.
anterior nares had been reopened. These results were
confirmed in work on the dogfish, (Mustelus), by Sheldon
(1911) and on the swellfish, (Spheroides), by Copeland
(1912). Sheldon closed the nares of the dogfish with
cotton plugs and, in 1914, I showed that when only one
nostril is thus plugged, the fishes turn persistently to-
ward the side of the open nostril. Such responses
indicate that in the seeking of food under normal con-
ditions, dogfishes, and probably other fishes as well, turn
PHYSIOLOGY OF OLFACTION 67
toward the side on which the concentration of odorous
particles is greater. The certainty of this operation
would increase in proportion as the nostrils of a given
fish are separated one from the other laterally. A good
example of an animal in which this condition reaches
its maximum is seen in the hammerhead shark in which
the nostrils, as well as the eyes, are carried on the re-
Records in per cent, of Turning Movements of three Dogfish under the follow-
ing successive Conditions: Normal, Left Nostril Occluded, Right Nostril
Occluded, Both Nostrils Open Parker, (1914).
States of Fishes
Turning Movements in Per Cents.
Left nostril occluded
Right nostril occluded
Both nostrils open
markable lateral projections that extend sidewise from
its head into the sea (Fig. 15).
The turning response of dogfishes under the condi-
tions mentioned in the preceding paragraph has a
striking resemblance toi the circus movements in the
tropic reactions of many of the lower animals and, were
it not that fishes are so highly organized, it might be
accepted at once as a response of that kind. The detailed
condition of such reactions is well illustrated by the
records in Table IV.
As a result of the evidence thus far accumulated, it
seems quite clear, contrary to the opinions expressed by
Nagel and others, that many fishes scent their food much
as air-inhabiting animals do and that they must be re-
68 SMELL, TASTE, ALLIED SENSES
garded as possessing powers of olfaction fairly compar-
able with those of the higher forms. This opinion is in
entire harmony with the well known fact that fishes,
especially sharks, can be drawn from a long distance by
ill smelling bait or by oily fish carcasses ground up and
thrown into the water as in the practice of chumming.
The extremely small amount of substance needed in these
operations agrees well with what is known of olfaction
among air-inhabiting vertebrates and reaches almost in-
finitesimal proportions as is indicated by the work of
Olmstcd (1918) on Amiurus.
The water-inhabiting stages of amphibians will doubt-
less be found to exhibit the same type of olfaction as
that seen in fishes. This is already clearly indicated by
the work of Copeland (1913) on the newt Diemyctylus
and of Kisser (1914) on tadpoles.
The opinion that fishes possess powers of olfaction
comparable with those of the air-inhabiting vertebrates,
though rejected by many of the older writers, has been
accepted in recent years by Baglioni (1913) and by
Luciani (1917). In fishes there can be no doubt that tho
stimulating material for the olfactory organs is carried
in the current of water that is passing more or less con-
tinuously through these parts. Since in air-inhabiting
vertebrates the stimulating materials are caught on the
watery mucous of the olfactory surfaces, it follows that,
as Durand (1918b) has recently declared, the olfactory
stimulus throughout the whole range of vertebrates is
material in a state of solution and not simply a gas or
a vapor. This conclusion is in agreement with the
opinion expressed many years ago by Johannes Mu'ller.
Henning (1916), some time since called attention to the
PHYSIOLOGY OF OLFACTION 69
possibility that odorous material may form with the ol-
factory mucous an emulsion rather than a true solution,
but this suggestion did not seem even to Henning to be
of much significance, for in other parts of his work he
refers repeatedly to the state of the stimulating material
as that of a solution and there appears to be no good
ground for assuming that such is not the case.
6. Fatigue and Exhaustion. It is well known that
the olfactory organs in man are quickly and easily fa-
tigued by continuous exposure to odorous materials.
Persons whose occupations lead them to work among
disagreeable odors soon become insensitive to these and
it has long been recognized that invalids are not affected
by the malodors that may come from their own bodies.
Although these conditions of irresponsiveness may be
due in part to central nervous states such as lack of at-
tention and the like, they are also dependent in part
on peripheral exhaustion. The effects of unpleasant
smells on the growth of guinea pigs has been tested very
recently by Winslow and Greenberg (1918). These in-
vestigators employed a pair of air-proof cages through
which were passed 1.5 cubic feet of air per minute
amounting to 4 liters of air per minute for each animal
in the test. Through one of these cages pure air was
circulated; through the other, air that had passed over
fresh moist faeces and that in consequence was impreg-
nated with a strong faecal odor. A total of 15 sets of
growing guinea pigs, including 261 animals, were sub-
jected to these conditions. In the first week of the tests
the animals supplied with faecal air did not grow as much
as the controls did, but in the second week they caught
up in weight with the controls and were thereafter in-
70 SMELL, TASTE, ALLIED SENSES
distinguishable from them. Thus the guinea pig, like
man, though sensitive to disagreeable odors in the be-
ginning, appears to become in the course of time entirely
inert to this form of stimulation.
To test the immediate effects of the continuous action
of odorous substance on the olfactory organ of man,
Aronsohn (1884a) determined the length of time certain
olfactory stimuli at full strength continued to call forth
sensation. Thus oil of lemon and oil of orange were
smelled by nine persons till the odors of these substances
could no longer be perceived The period necessary to
bring about this obliteration of sensation varied from
2.5 minutes to 11 minutes with an average of 3 minutes.
A 0.2 per cent solution of cumarin in water was smelled
for from 1.75 to 2.3 minutes after which it was no longer
sensed. Thus olfactory exhaustion under strong stimu-
lation is accomplished in a very few minutes. The re-
covery of excitability is apparently equally rapid and
may be accomplished in as short a time as from 1 to 3
minutes though complete recovery probably requires a
Zwaardemaker (1895) tested fatigue in another way
and determined by means of his olfactometer the in-
crease in minimum stimulation as the olfactory organ
gradually approximated exhaustion. During a continu-
ous stimulation of known intensity the minimum stimu-
lus was from time to time determined and was found to
increase steadily. Two substances, benzoin and rubber,
at two different strengths were tested (Fig. 16). Ben-
zoin induced fatigue more rapidly than rubber and of
the two concentrations employed for each substance the
PHYSIOLOGY OF OLFACTION
stronger in each instance called forth fatigue more
quickly than the weaker.
Some persons are absolutely devoid of true olfaction,
a condition which, as already pointed out, is attendant
upon certain deficiencies in the essentials of the olfac-
tory apparatus and which is designated as anosmia.
10 20 30 40 50 60 70 80 90 Sec*
FIG. 16. Curves of olfactory exhaustion produced by the action of benzoin of 9 and
of 3.5 olfacties and &by rubber of 14 and of 10 olfacties, acting for different periods.
The threshold values in olfacties are marked on the ordinates and the duration of
stimulation in seconds on the abscisste. After Zwaardemaker, 1895, Fig. 22.
This state may be congenital or acquired and acquired
anosmia may be either permanent or temporary. Some
forms of anO'Smia, like color-blindness, are probably in-
heritable Glaser (1918). Of considerable interest from
a theoretical standpoint are the cases of partial olfac-
tory defects. Winkler noted a patient who was quite
incapable of smelling benzoin though he easily recognized
musk and another who was just the reverse of the first
72 SMELL, TASTE, ALLIED SENSES
one. Blakeslee (1918) has recorded similar cases in
relation to the odor of verbena flowers. Probably many
persons are defective in this respect though their defects
may not have been serious enough to have attracted
attention and record.
Temporary partial anosmia may accompany certain
diseases or may be induced by the application to the
olfactory surfaces of anesthetizing drugs. Cocaine has
been used in this way by a number of investigators, in-
cluding Zwaardemaker, but without very clearly defined
results. Zwaardemaker observed that temporary anos-
mia induced in this way was preceded by a brief period
of increased sensibility or hyperosmia. Subsequently
Keuter (1900) found that cocaine was also followed by
hyperosmia. Roljett (1899) produced a complete an-
osmia by the use of gymnemic acid after which different
olfactory sensa.tions returned at different intervals.
7. Qualities of Odors. The qualities of odors ap-
pear to be almost innumerable. When we attempt to
name on odor, we almost always designate it by the body
from which the odorous material emanates like the smell
of heliotrope, of onion, of rubber, and so forth. With
tastes, as we shall see later, there are at least four
clearly marked qualities, sweet, sour, bitter, and salty.
The first three of these are general terms connected in
no necessary way with the substances associated with
them as stimuli, and we are continually finding new sub-
stances whose tastes are some one of these three. The
odors of new substances, on the other hand, are almost
certain to be individual and novel and to agree with odors
already known only in a most general way. Thus odors
have a certain historical value and get their names after
the introduction of the substances with which they are
PHYSIOLOGY OF OLFACTION 73
associated; the smell of illuminating gas was not a gen-
erally known odor till this material was brought into
common use. Should it be abandoned commercially, its
odor would cease to be a part of common human sensa-
tion. In consequence of economic changes many odors
of trade articles, of kitchen products, and the like have
disappeared from the list of human sensations and many
new ones have come in. Yet notwithstanding this rela-
tively rapid evolution in the field of olfaction, the organ
of smell seems to remain the same ; it gives up old forms
of stimulation and takes on new ones in a way that is
almost incredible. As a result of these peculiarities of
the olfactory organ the classification of odors has
proved to be a most perplexing problem and has
resulted in most instances in what seem to be extremely
Haller and particularly Linnaeus proposed systems of
odors that have formed the bases for many of the modern
classifications such as the one given by Zwaardemaker
(1895). In this odors are arranged in nine general
classes each of which may contain two or more sub-
divisions. These nine classes are briefly as follows :
1. Etherial odors; three subdivisions: odors of fruits,
2. Aromatic odors ; five subdivisions : odors of camphor,
cloves, lavender, lemon, bitter almond.
3. Balsamic odors ; three subdivisions : odors of flowers,
violet, vanilla, cumarin.
4. Ambrosial odors ; two subdivisions : odors of am-
5. Alliaceous odors ; three subdivisions : odors of hydro-
gen sulphide, hydrogen arsenide, chlorine.
74 SMELL, TASTE, ALLIED SENSES
6. Empyreumatic odors; two subdivisions: odors of
roast coffee, benzole.
7. Caprilic odors; two subdivisions: odors of cheese,
8. Repulsive odors; two subdivisions: odors of deadly
9. Nauseating odors; two subdivisions: odors of car-
A survey of this classification shows at once that more
or less of it is associative and subjective and hence ar-
tificial, for what may be repulsive to one person may be
just the reverse to another. It is, therefore, not sur-
prising that some of the recent students of this subject,
as for instance Henning (1916), have advised the com-
plete abandonment of such arrangements and have
sought to establish by a thorough re-testing of odors an
impersonal and reasonable classification. As the re-
sult of an extended and judicious re-examination of odors
Henning has come to the conclusion that they fell into
six fundamental classes as follows :
1. Spicy odors, such as those of fennel, sassafras oil,
anise, and cloves.
2. Flowery odors, such as those of heliotrope, cumarin,
and geranium oil.
3. Fruity odors, such as those of oil of orange, citro-
nella, oil of bergamot, and acetic ether.
4. Resinous or balsamic odors, such as those of tur-
pentine, of Canada balsam, and of eucalyptus oil.
5. Burnt odors, such as those of tar and pyridine.
6. Foul odors, such as those of carbon bisulphide and
of hydrogen sulphide.
Although each of these six classes, according to
PHYSIOLOGY OF OLFACTION
Henning, is represented by a number of odors, it is not ab-
solutely separated from the others, but between any pair
of them there are numerous odors that assume interme-
diate positions. The six classes, however, are the striking
predominant elements in this complex and are in no sense
submerged in the general array of odors.
Henning has tried to make clear his idea of the rela-
tions of these six classes by imagining them located one
FIQ. 17. Olfactory prism. After Henning, 1916, Fig. 4.
at each corner of a three-sided prism which he calls the
olfactory prism (Fig. 17). From each corner of this
prism lines may be imagined to pass out to the other
corners ; these lines traverse either the edges of the prism
or pass over its faces and mark the positions of all in-
termediate odors. Thus all odors, be they fundamental
or intermediate, find places on the surface of the prism.
Relations indicated by lines within the prism and con-
necting any two points on its surface indicate only
mixed odors. Thus by means of a figure of three dimen-
sions Henning brings into clear view the relations
he conceives to exist between the six fundamental odors,
76 SMELL, TASTE, ALLIED SENSES
their intermediates and mixtures. So far as an arrange-
ment of odors is concerned the clarity of Henning's
scheme is at once its most attractive and most suspi-
8. Chemical Relations of Odors. The scientific
value of any classification of odors will depend upon the
success with which such a classification brings the odor-
ous substances as stimuli into relation with the receptor.
A satisfactory classification ought to make evident the
number of elements or components concerned in olfac-
tion. That olfaction is made up of a number of compo-
nents is far from established, but what may be called
the component theory of olfacton is generally assumed
by the majority of writers on this subject Zwaardemaker
(1895). That the classification outlined by Zwaarde-
maker shows very little of this feature is readily admit-
ted even by this author himself. Quite aside from the
fact that it may include irritants as well as true odorous
substances, its classes do not stand up well under experi-
mental test. Nagel (1897) tested this question in an
investigation of the odors of vanillin and cumarin.
These two substances, according to Zwaardemaker 's
classification, belong not only to the same class of bal-
samic odors but to the same subdivision, the vanilla odor.
They ought, therefore, to show considerable olfactory
similarity. Nagel attempted to test this relationship by
ascertaining whether the temporary exhaustion of the ol-
factory organ by one of these substances would influence
its receptive capacity for the other. To carry out this
he prepared an aqueous solution of the two substances
in such proportions that the smell of only vanillin could
be recognized. He then exhausted the olfactory organ
PHYSIOLOGY OF OLFACTION 77
for vanillin by smelling for a long time a pure solution
of this material. On testing now the solution containing
the mixture of substances, it was found to smell only
of cumarin. Thus the exhaustion of the olfactory sur-
face for vanillin did not prevent stimulation by cumarin.
The placing of these two substances in the same subdi-
vison is, therefore, obviously artificial.
Similar evidence as to the artificiality of Zwaarde-
maker's classifications had also been obtained from the
study of persons suffering from partial anosmia and from
neither this line of investigation nor from that dealing
with partial exhaustion has there come any special jus-
tification of the conventional olfactory groupings.
Yet it is admitted on all sides that olfaction is essen-
tially a chemical process. And, as a matter of fact,
some progress has been made in discovering relations
between chemical structure and olfactory sensation.
This isi not necessarily of a general nature, but seems/
usually to be limited to narrow ranges. Thus among the
alcohols Passy (1892c) has discovered that the olfactory
potency increases progressively in passing over this se-
ries from methyl to amyl as shown in Table V.
Ba,ckman (1917c) has likewise determined that ini
the methylbenzene series olfactory acuity for benzene,
toluene, xylene, cumene, and durene increases as the sub-
stitute methyl group increases.
Changes in the quality of odors also follow some natu-
ral series of organic compounds as has been pointed out
by Hay craft (1900) in the following etherial salts.
Ethyl acetate with acetic etherial odor.
Propyl acetate with acetic odor and slight flavor.
Butyl acetate with slight acetic odor and pineapple flavor.
78 SMELL, TASTE, ALLIED SENSES
Amyl acetate with no acetic odor but well marked pine-
Ethyl acetate and amyl acetate have entirely distinct
odors, but when propyl acetate and butyl acetate are
taken into consideration the four compounds form a se-
ries in which there is a transition in odors corresponding
Estimated potencies of alcohols, Passy (1892c).
Alcohol Estimated Potency
to the changes in chemical structure. Other series of
homologues, however, such as the one tested by Huyer
(1917), analine, o-, m-, and p-toluidine, xylidine, and cu-
mioline, show no such relations.
Not a few investigators have suggested that the odors
of many substances depend upon the number and ar-
rangement of certain chemical radicals contained within
the odorous molecule. Such radicals are commonly
called osmophoric groups. Perhaps one of the most
considerable studies of this kind was that carried out by
Cohn (1904), but without commensurate results. The
most recent and ambitious of these attempts is by Hen-
ning (1916) whose classification of odors has already been
Henning's studies on the relations of odors to chemical
constitution have to do almost entirely with the aromatic
compounds, though there is no reason to believe that his
generalizations, if true, may not be extended eventually
PHYSIOLOGY OF OLFACTION 79
t . f
to the aliphatic series. He abandons the idea that spe-
cial odors are to be associated with particular osmopho-
ric groups. In odors these groups are significant, not
because of the structure they themselves possess, but
because of the positions they may occupy on the benzene
ring. Osmophoric groups are such as the hydroxyl, al-
dehyde, keton, ester, nitro, and nitril groups. None of
these, however, is associated with a particular odor, but
any one may be the occasion of odor, if it occupies an
appropriate place on a benzene ring. The position on the
ring not the particular radical, according to Henning, is
the determining factor so far as odor is concerned.
Henning is further convinced that in a general way
types of chemical constitution can be indicated for the
six groups of odors that he was able to distinguish
(Fig. 18). Thus the class of spicy odors is represented
by compounds in which the osmophoric groups are in
para-position (Fig. 18a), as in anisaldehyde. In the
flowery odors the osmophoric groups are in the meta- or
the ortho-positions (Fig. 18b), as in tuberon. In the
fruity odors the groups are forked (Fig. 18c) as in cit-
ral. In the resinous odors the groups are within the
ring (Fig. 18d) as in pinene. In the burnt odors the
ring is smooth (Fig. 18e) as in pyridin, and in the foul
odors the ring is fragmentary (Fig. 18f ) as in cacodyl.
In this way each class of odors is associated with a spe-
cial feature in the constitution of the molecule though
not necessarily with a particular osmophoric group. In-
termediate odors are due to combinations of groupings
which partake of the nature of the two classes between
which the intermediate lies. Thus vanillin has an odor
between spicy and flowery and its three osmophoric
80 SMELL, TASTE, ALLIED SENSES
groups (Fig. 18g) are attached so as to represent both
the para-position (spicy) and the ortho-position (flow-
ery). By this ingenious system Henning has attempted
to connect odor with chemical constitution and though
Fio. IS. Generalized diagrams of the molecular structure of the six classes of aromatic
bodies that serve as olfactory stimuli according to Henning (1910); a, for spicy odors; b. for
flowery odors; c, for fruity odors; d, for resinous odors; e, for burnt odors; f, for foul odors;
and g, for au intermediate odor between spicy and flowery.
the attempt is avowedly fragmentary and may be open
to much subsequent modification, it gives promise of
the solution of a problem that heretofore has been
9. Inadequate and Adequate Stimuli. Inadequate
olfactory stimuli are apparently very few in number and
not well known. Thermal stimuli when applied to the
PHYSIOLOGY OF OLFACTION 81
olfactory organs are said to call forth no sensations of
smell, and Valentin's statement that mechanical stimuli
will produce unpleasant olfactory sensations has not been
confirmed. Aronsohn (1884b), after filling the nasal
cavity with warm physiological salt solution led a direct
electric current through this cavity with the result that
certain obscure sensations were produced depending upon
whether the anode or the cathode was within the nose.
With the anode in the nose a sensation was called forth
on opening the circuit; with the cathode in the nose on
closing it. There was, however, no evidence to show
that these effects were not due to a stimuation of tri-
geminal endings instead of olfactory endings. Althaus
in 1869 recorded as the outcome of electrical stimulation
a phosphorous-like smell in a patient suffering from
double trigeminal paralysis. Apparently the electric
current is a true inadequate stimulus for the olfactory
organ, but its peculiarities are very incompletely under-
stood. Aside from this and the effects from solutions
as described by Veress, inadequate olfactory stimulation
seems not to exist.
The adequate olfactory stimulus for both water-in-
habiting and air-inhabiting vertebrates is a solution in
contact with the olfactory hairs and perhaps formed in
part within these bodies. The solvent is probably first
the olfactory mucous which receives the solute from the
current of water or of air that passes over its outer sur-
face. This watery solvent, which from its nature must be
almost universal in its dissolving power, passes the solute
on to the olfactory hairs whose capacity as receptors is
probably limited by their lipoid composition. Only those
substances that are soluble in lipoids can be taken up by
82 SMELL, TASTE, ALLIED SENSES
the hairs, a process that must precede the initiation of the
olfactory nerve-impulse. The solute may be any one
of an immense variety of substances whose primary char-
acteristics are that they are not only soluble in water but
also in oil. The amount of these substances necessary
for olfaction even in the case of the least odorous of
them is very small and in that of the most odorous in-
credibly small. The amounts that are usually estimated
for olfaction are those contained in what is believed to
be the minimum volume of water or of air necessaiy
for stimulation, but of the very minute amount of odor-
ous substance contained in this volume only a veiy small
fraction of it can reach the olfactory hairs. Much must
be carried away in the general current or left stranded
on non-olfactory portions of the nasal surfaces. Whether
the olfactory hairs can concentrate this material or not
remains to be ascertained, but even assuming that they
can, the effective concentration must be of an extremely
The substances thus brought in solution into the ol-
factory hairs must initiate those nervous changes that
eventually produce the olfactory sensations. There
ought, therefore, to be some relation between these sub-
stances and the resulting sensations. It is generally
assumed that the substances that act as olfactory stimuli
fall into classes associated with corresponding classes
of sensations. As already indicated this conception may
be called the component theory of olfaction, and if we
assume, for instance, that the six classes of odors dis-
tinguished by Henning are separate classes, a view that
Kenning, however, opposes, then these classes would
PHYSIOLOGY OF OLFACTION 83
represent the olfactory components that physiologists
have sought for so long.
The very existence of partial anosmia implies olfac-
tory components the inactivity of one of which is ac-
countable for the partial defect. But such cases are too
little known to admit of clear interpretation. Thus
Aronsohn's observation (1886) that partial anosmia pro-
duced by the exhaustion of the nose through ammonium
sulphide leaves that organ sensitive to etherial oils but
insensitive to hydrogen sulphide, hydrochloric acid and
bromine, may be a differential effect between true odors
(olfactory endings) and irritants (trigeminal endings),
and not between groups of true odors. Nevertheless it
must be in this direction that an experimental analysis of
the general problem of olfaction will eventually proceed.
From this standpoint the condition presented by
mixed odors is of significance. At least two classes of
odor mixtures are to be distinguished, one spurious and
the other real. Spurious mixed odors are those in which
the gases or vapors act chemically on each other and thus
produce a third substance which may or may not have
an odor of its own. Thus ammonia and acetic acid both
stimulate the nose, but when mixed they possess no odor
for they combine to form odorless ammonium acetate.
Obviously such instances are not, accurately speaking,
instances of mixed odors. On the other hand there are
many pairs of odorous substances in which one member
does not act upon the other chemically and consequently
in which the two are left to act independently on the ol-
factory receptors. Such double stimuli, from the stand-
point of the component theory might be expected to excite
two sensations, but apparently this is not always the
84 SMELL, TASTE, ALLIED SENSES
case. If in a pair of such odors one is much stronger
than the other, its smell dominates completely. If, how-
ever, the two odors are closely balanced a true odor may
result which in quality is said to be unlike that of either
component. Novel odors of this kind may be produced,
according to Aronsohn (1886), by such combinations as
cologne water and oil of orange, cologne water and oil of
lemon, oil of bergamot and oil of orange, and so forth.
The condition that thus produces a noval odor is one of
considerable delicacy and may be easily upset by the
greater exhausting effect of one or other of the components
thus allowing the less exhausted member to assert itself
and to call forth its own peculiar sensation. The presence
of a sensation different from those of the pair of stimuli
producing that sensation, might seem to be a condition
adverse to the component theory, but it must be remem-
bered that in vision, in which the component conception
is fundamental, an exact parallel occurs. Thus when a
pure orange light is mixed with a pure green light, there
may result a sensation of yellow that is wholly unlike
that appropriate to either member of the combination,
and that, as a matter of fact, may be indistinguishable
from a sensation of yellow produced by a pure yellow
] ight. Thus in accepting the component theory of sensory
activity it must be admitted that two stimuli together
may excite a receptor in precisely the same way as a
third and entirely different stimulus may do. The exist-
ence of a novel olfactory sensation due to the simultane-
ous activity of two independent stimuli is therefore, no
serious obstacle to this theory.
The condition of double olfactory stimulation that
has just been described must not be confused with a kind
PHYSIOLOGY OF OLFACTION 85
of double stimulation that has been much studied. Val-
entin observed that when ether and balsam of Peru were
smelled at the same time one by one nostril and the other
by the other nostril, the odors are perceived not together
but alternately and Valentin believed that there was
a sensory conflict here as in vision, when one eye is
directed to a field of one color and the other eye to one
of another color. Aronsohn (1886) noted a similar con-
flict between the smell of camphor and that of oil of
lemon. He also discovered that under similar circum-
Pairs of neutralizing odors (Zwaardemaker, 1895, p. 168).
Pairs of odorous bodies Neutralizing Strength
Cedarwood and rubber 2.75 : 14
Benzoin and rubber 3.5 : 10
Paraffin and rubber 8.5 : 14
Rubber and wux 14 :28
Rubber and balsam of Tolu 14 :70
Wax and balsam of Tolu 40 :90
Paraffin and wax 10 :20
stances one smell could overcome another. Thus the
smell of camphor was neutralized by the smell of pe-
troleum, cologne water, oil of juniper and so forth.
This question was investigated much more fully by
Zwaardemaker (1895) who employed for this purpose
his double olfactometor. By this means it was compara-
tively easy to- balance odors and then lead one into one
nasal cavity and the other into the other cavity. In this
way complete neutralization could be attained with great
accuracy. Table VI gives a list of neutralizing pairs of
odors and the intensity in olfacties at which Zwaarde-
maker found neutralization to occur.
86 SMELL, TASTE, ALLIED SENSES
It is needless to say that since in this form of double
stimulation one stimulus is applied to one olfactory organ
and the other to the other organ, the phenomenon of neu-
tralization cannot depend upon the chemical action of
one odor upon the other, for the odorous materials are
not allowed to mingle. The fact that they are separately
applied to different receptors shows that this type of
conflict and of neutralization must have a central origin.
10. Olfactory Eeflexes. In discussing the relations
of the two categories of nasal stimuli, irritants and true
odors, Frohlich attributed reflex action to the first but
not to the second, and it is true that nasal irritants
almost invariably call forth vigorous respiratory re-
sponses, such as sneezing, whereas true odors are seldom
followed by reactions of a marked kind. Pawlow, how-
ever, has pointed out the great importance of true odors
in exciting and, in a way, in controlling the whole chain
of digestive secretions, a process just as significantly
reflex as sneezing but not so easily observed. Both
classes of stimuli, then, are followed by abundant and
important reflexes, but in one class these are of a kind
easily noticed, in the other they are more hidden.
Although the olfactory organs in man are unques-
tionably concerned with the odors of the food that is
being masticated, they are much more concerned with
the odors of the environment. From this standpoint the
olfactory organs are properly classed as distance-recep-
tors or receptors affected by stimuli that emanate from
more remote points in the surroundings. In consequence
our olfactory sensations are in a way projected into the
exterior and we seek, avoid, or recognize the distant body
by its odor. The smell of a skunk is unquestionably a
PHYSIOLOGY OF OLFACTION 87
protective odor in that it implies that it can be sensed
by other animals that will thereupon avoid its source.
The great delicacy of olfaction among the higher animals
by which they can scent the hunter is well known. Other
odors have much to do with sexual activities whereby one
sex is led to find the other or is otherwise excited to ac-
tivity. But the prime service of olfaction is in the quest
of food. From the fishes to the mammals olfaction
serves as a means of discovering hidden or remote food
and in this respect it is a highly significant sense for the
direction of locomotion. In man and other microsmatic
forms much of the keenness of olfaction has disappeared
and yet the high development of this sense in our an-
cestry has left such a profound impression on the
organization of our central nervous apparatus that we are
often surprised by the power of our olfactory associations.
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VOMERO-NASAL ORGAN OR ORGAN OF JACOBSON.
Contents. 1. Vomero-nasal Organ in Man. 2. Com-
parative Anatony. 3. Histology. 4. Adjacent Parts.
5. Function. 6. Bibliography.
VOMERO-NASAL Organ in Man. In early infancy all
human beings show traces of a pair of organs that are
without doubt homologues of the vomero-nasal organs of
the lower vertebrates. Each vomero-nasal organ in the
new-bora babe is a short tubular structure from a half
to two and a half millimeters long and lodged in the lower
anterior portion of the nasal septum. The organ opens
into the nasal cavity by a minute pore on the free sur-
face of the septum not far from its ventral border and
onlv a short distance inward from the external naris.
The tubular part of the organ extends posteriorly from
this minute pore and ends blindly at a point somewhat
higher than the level of the pore itself.
In early human embryos the pore of the vomero-nasal
organ can be easily identified on the median face of the
nasal chamber just within the anterior naris (Fig. 19). In
adults the organ, though commonly present, may disap-
pear completely. When present it occurs near the ven-
tral margin of the nasal septum (Fig. 20). Kolliker
(1877) states that it may vary in length from two to
seven millimeters and Anton (1895) gives as the extremes
2.2 millimeters and 8.4 millimeters. As seen in trans-
verse section it has the appearance of a tube flattened in
the plane of the nasal septum. Its lateral wall is cov-
ered with an epithelium that resembles histologically the
respiratory epithelium of the nasal cavity. This lateral
epithelium may even be ciliated. The median wall is cov-
ered with an epithelium much like the olfactory epithelium
of the nose except that differentiated olfactory cells are
apparently not present. The cavity of the organ is
sometimes obliterated by excessive
epithelial growth and calcareous con-
cretions may occur in its walls. As it
appears to be without nervous con-
nections, the vomero-nasal organ in
man is probably entirely rudimentary.
2. Comparative Anatomy. A vo-
mero-nasal organ has been recog-
nized for some time past in all classes
of vertebrates except the fishes, but,
according to Gawrilenko (1910), this
group too must be admitted to have at
least the f oreshadowings of such an
as the sharks and rays each oltactory
sac is divided into two compartments
with separate innervation and these two compartments
may be supposed to correspond one to the vomero-nasal
organ and the other to the olfactory organ proper. This
double character of the olfactory apparatus is also seen in
other fishes. Thus in the development of the olfactory sac
of the salmon Gawrilenko has shown that this organ
includes two sensory thickenings or placodes, a median
one and a lateral one. These two placodes can be
traced into the adult where they are said to give rise to
a median olfactory area and a lateral area. The median
F IG. 19. Lateral view
-.-. . , ... ^ of the head of a human
Even in such primitive iorms embryo showing the pore
(v) of the vomero-nasal
After HIS, isss,
SMELL, TASTE, ALLIED SENSES
area is believed to correspond to the vomero-nasal organ
of the higher vertebrates and the lateral area to the true
olfactory receptor of these forms.
In some amphibians the distinction between a lateral
and a median organ is much more evident than in fishes
(Fig. 21). The lateral organ is the one that conducts the
Fio. 20. Diagram of the median face of the left nasal ravity of man ; the small circle marks
the position of the vomero-nasal organ in the nasal septum.
newly established air current from the external naris to
the choana and hence corresponds to the olfactory organ
proper. The median cavity is less involved in this cur-
rent and is believed to represent the vomero-nasal organ.
In certain sauropsida such as the alligators and tur-
tles the vomero-nasal organ has been said to be at best
only poorly developed, though so far as turtles are con-
cerned this opinion is not shared by one of the most
recent workers, McCotter (1917). In birds the organ
is claimed to be entirely absent, but in lizards and in
snakes it is highly differentiated (Fig. 22). Here the
olfactory apparatus consists of a well-developed organ
of smell located in the respiratory passage and an
entirely independent vomero-nasal organ. The latter,
in the form of a blind sac, opens into the cavity of the
mouth. This peculiarity is probably dependent upon the
growth of the hard palate in reptiles whereby a new
adjustment between the nasal cavity and the mouth is
In mammals the vomero-nasal organ also shows much
diversity. It is apparently best developed in the lower
Fia. 21. Transverse section of the snout of a young frog snowing the partial division
of the nasal cavity into a lateral or olfactory portion (o) and
a median or vomero-nasal portion (v).
forms, such as the Australian duckbill Ornithorhynchus,
and it is rudimentary in such groups as the primates
including man. In general it has the form of a blind
sac that opens usually by means of the naso-palatine duct
(Stenson's duct) into the mouth, a relation that is prob-
ably reminiscent of its original connection with the
primitive choana of which the naso-palatine duct may
be regarded as a trace. Less commonly it opens directly
by its own duct into the nasal cavity. This condition
obtains in certain rodents such as the rabbit, guinea pig,
rat, and mouse, and in certain primates including man.
SMELL, TASTE, ALLIED SENSES
In all these higher vertebrates the olfactory organ proper
corresponds to the lateral component of the pair of
organs in the lower forms and the vomero-nasal organ
to the median member of this group (Fig. 23).
3. Histology. The vomero-nasal organ of the dog
and the cat, as described by Bead (1908), is a tubular
organ whose transverse
section is circular in out-
line near its opening
and crescentic or kidney-
shaped throughout its
greater extent. Its median
wall may be two to three
times as thick as its lateral
wall. This thickened por-
tion, which has been
observed by numerous
workers in a variety of
mammals, is similar in
Fin. 22. Transverae section of the head Cellular Composition to the
of a snake embryo (Agkistrodon) showing , . -JIT c
the nasal canal proper (n) and the large OltaCtOry epithelium Ol
vomero-nasal (v) organ opening on the ' .
roof of the mouth. Preparation by Mr. tll6 11OSC aild IS 111 StrOllg
F. B. Manning.
contrast with the lateral
thin wall which resembles respiratory nasal epithelium.
Read has shown that the vomero-nasal organ of the cat
and the dog is like the olfactory region of the nose in that
it receives nerve fibers from two sources, the olfactory
nerve and the trigeminal nerve.
As early as 1892 von Brunn showed that the sense
cells of the vomero-nasal organ of the sheep were con-
nected with nerve fibers in exactly the way they were in
the olfactory region proper and he assumed, probably
with correctness, that these fibers belonged to the olfac-
tory nerve (Fig. 24). These observations were confirmed
by all subsequent workers including von Lenhossek (1892)
in the rabbit, Retzius (1894) in the snake, Ramon y Cajal
(1895) in the rat, and Read (1908) in the kitten.
Retzius showed that in the snake those nerve-fibers that
were connected with the sense cells in the vomero-
nasal organ mingled with the bundle of fibers from the
olfactory region of
the nose and thus con-
firmed von Brunn's
suspicion that vo-
true olfactory fibers.
pointed out that at
least in the rabbit the
sense cells were not
limited to the thick-
;ened face of the vo-
inero-nasal organ, as
had been maintained heretofore, but were found upon the
opposite tlu'n face of the organ as well This observation
was confirmed on the rat a few years later by Ramon y
Cajal. Hairlike terminations on the vomero-nasal sense
cells, such as those that had been found in the olfactory
cells, were sought for by a number of investigators
but only traces of these structures could be found (von
Brunn, Retzius, Read), probably because of the ease
with which they are Destroyed in the preparation of
Von Lenhossek in 1892 not only confirmed von
FIG. 23. Transverse section of the nasal
septum of a young cat showing the vomero-
nasal organ (v), its.cartilage (c), and the nasal
SMELL, TASTE, ALLIED SENSES
Brunn's observation that the vomero-nasal sense cells
were directly connected with nerve fibers, but he also
pointed out that in the Jacobson organ of the foetal rab-
bit free-nerve terminations occurred. These free termi-
nals in some instances reached the receptive surface of
the epithelium where they ended in slight knobs. Similar
endings were recorded for the
rat by Bamon y Cajal (1895).
Von Lenhossek was unable to
decide definitely whether these
terminals belonged to the olfac-
tory or to the trigeminal nerve.
Nor is this question definitely
settled now, though, judging
from the conditions met with
in the olfactory organ of
the nose, it is highly prob-
able, as Eead concludes, that they belong to the trigem-
inal nerve. Admitting this to be the case, the innervation
of the vomero-nasal organ would agree in all particulars
with that of the olfactory organ proper. It is quite
clear from the studies of Brookover (1917) on the ner-
vus termmalis as well as from those of Larsell (1918)
that the relations of this nerve to the vomero-nasal organ
are merely incidental; the terminal nerve is in no sense
especially connected with the organ of Jacobson.
4. Adjacent Parts. In many of the higher verte-
brates the vomero-nasal organ is contained within a more
or less complete capsule of cartilage, the Jacobson car-
tilage (See Fig. 23). In the cat this capsule, according
to Read, is complete anteriorly and incomplete posteri-
orly; in the dog it is incomplete throughout its whole
Fio. 24. Epithelium from the
vomero-nasal organ of the sheep
showing the receptive cells impreg-
nated by the Golgi method. After
von Brunn, 1892, Plate 30, Fig. 12.
VOMERO-NASAL ORGAN 99
length. The vomero-nasal organ of these forms has com-
monly associated with it a considerable amount of caver-
nous tissue. This tissue, which was long ago identified
in nasal organs by Klein (1881a, 1881b), is so disposed
that in connection with the surrounding cartillage and
other parts, it may serve as a means of changing in no
small degree the volume of the organ.
5. Function. Concerning the function of the vomero-
nasal organ almost nothing is known. Von Mihalkovics
(1898) found that after burning out the naso-palatine
duct and more or less of the vomero-nasal organ
in the cat and in the rabbit, the appropriation of food
by these animals was not interfered with, but it is hardly
to be expected that so crude an experiment as this would
yield significant results. Kolliker emphasized the fact
that, at least in mammals, the connection between the
vomero-nasal organ and the exterior is so narrow and
indirect that it seems almost impossible that there should
be any transfer of material from the exterior to the inte-
rior of the organ as, for instance, is implied in olfaction.
He, therefore, suggested that the vomero-nasal organ
was concerned with testing the animal's own juices as rep-
resented by the secretions from this organ. But the
vomero-nasal organ, particularly in mammals, is inti-
mately associated with much cavernous tissue whose
change in volume may be concerned with its filling and
emptying. Thus it is quite possible that oral or nasal
juices may be sucked into the vomero-nasal organ and
discharged from it as has recently been maintained by
Broman (1918). Henning (1916) has suggested that the
organ is concerned with water olfaction as contrasted
with air olfaction, but according to an unpublished obser-
100 SMELL, TASTE, ALLIED SENSES
vation of Mr. H. E. Hamlin air is often found in the
vomero-nasal organs of freshly killed mammals, and this
observation when taken in connection with the work of
Broman supports the hypothesis already advanced by
many investigators (P. and F. Sarasin, 1890; Seydel,
1895; Gaupp, 1900) that these organs are subsidiary
olfactory receptors, an opinion that, while it lacks com-
plete experimental proof, is abundantly supported by the
finer structure of the parts concerned.
ANTON, W. 1895. Beitriige zur Kenntnis dos Ja^obson't-chen Organes bei
Erwachsenen. Zeitschr. Eeitk., Bd. 16, pp. 355-372.
BROMAN, I. 1918. Om Jacobsonska Organets konstruktion om funktion.
Lunds Univ. Arsskrift, N. F., Avd. 2, vol. 14, No. 4, 40 pp.
BROOKOVER, C. 1917. The Peripheral Distribution of the Nervus termin-
alis in an infant. Jour. Comp. Neurol., vol. 28, pp. 349-360.
VON BRUNN, A. 1892. Die Endigung der Olfaetoriusfasern im Jacob-
son'schen Organe des Schafes. Arch mik. Anat., Bd. 39, pp. 651-652.
GAUPP, E. 1900. Das Chondrocranium von Lacerta agilis. Anat. Hefte,
Arb., Bd. 15, pp. 433-595.
GAWRILENKO, A. 1910. Die Entwickeltmg des Geruchsorgan bei Salmo
salar. Anat. Anz., Bd. 36, pp. 411-427.
His, W. 1885. Anatomic menschlicher Embryonen. Ill Zur Geschichtc
der Organe. Leipzig, 260 pp.
HENNING, H. 1916. Der Geruch. Leipzig, 533 pp.
KALLIUS, E. 1905. Geruchsorgan. Bardeleben, Handb. Anat. Menschcn,
Bd. 5, Abt. 1, Teil 2, pp. 115-242.
KLEIN, E. 188 la. Contribution to the Minute Anatomy of the Nasal
Mucous Membrane. Quart. Jour. Mic. Soi., vol. 21, pp. 98-1 13>.
KLEIN, E. 1881b. A Further Contribution to the Minute Anatomy of
the Organ of Jacobson in the Guinea-pig. Quart. Jour. Mic. Sri.,
vol. 21, pp. 219-230.
KOLLIKER, A. 1877. Ueber des Jacobsonsche Organ des Menschen. Graf.
VON LENHOSSEK, M. 1892. Die Nervenurspriinge und -Endigung^n im
Jacobson 'echen Organ des Kaninchens. Anat. Anz., Bd. 7, pp. 628-635.
LARSELL, 0. 1918. ' Studies on the Nervus terminalis: Mammals. Jour.
Comp. Neurol., vol. 30, pp. 1-68.
VOMERO-NASAL ORGAN 101
McCoTTER, R. E. 1917. The Vomero-nasal apparatus in Chrysemyg punc-
tata and Rana catesbiana. Anat. Rec., vol. 13, pp. 51-67.
VON MIHALKOVICS, V. 1898. Nasenhohle und Jacobsonches Organ. Anat.
Hefte. Art., Bd. 11, pp. 1-107.
RAMON Y CAJAL, S. 1895. Les nouvelles Idees sur la Structure du
Syst&me Nerveux. Paris, '201 pp.
READ, E. A. 1908. A Contribution to the Knowledge of the Olfactory
Apparatus in the Dog, Cat, and Man. Amer. Jour. Anat., vol, 8,
RETZIUS, G. 1894. Die Riechzellen der Ophidier in der Riechschleiim-
haut und im Jacobson'schen Organ. Biol. Unters. N. F., Bd. 6, pp. 48-51.
SABASIN, P. und F. SAEASIN. 1890. Zur Entwicklungsgeschichte Und
Anatomie der ceylonesischen Blindwiihle Ichthyophis glutinosus L.
Ergeb. naturw, Forsch. Ceylon, Bd. 2, pp. 1-263.
SEYDEL, O. 1895. Ueber die Nasenhohie und daa Jacobsonsche Organ
Der Amphibien. Morph. Jahrb., Bd. 23, pp. 453-543.
SYMINGTON, J. 1891. On the Nose, the Organ of Jacobson, and the
Dumb-bell shaped Bone in Ornithorhynchus. Proc. Zool. Soc. London,
1891, pp. 575-584.
ZUCKERKANDL, E. 1910. Das Jacobsonsche Organ. Ergeb. Anat. Ent-
wick., Bd. 18, pp. 801-843.
THE COMMON CHEMICAL SENSE.
Contents. 1. Common Chemical Sense in Man. 2. In
Lower Vertebrates. 3. Nerve Terminals. 4. Relation
to Other Senses. 5. Bibliography.
1. COMMON Chemical Sense in Man. It was long 1
ago made clear by Frohlich that on the nasal surfaces
of man there were two systems of receptors that could
be stimulated by gaseous or vaporous materials: olfac-
tory cells representing the olfactory nerve, whose stim-
uli, delicate perfumes and odors, call forth few observable
responses, and free-nerve terminals probably represent-
ing the trigeminal nerve, whose stimuli, irritants for the
most part, are usually followed by vigorous reactions
such as sneezing. This distinction has been generally
accepted among physiologists, but it has not been so
clearly seen that the receptors for irritants are found in
other parts of the body than the nose and that they rep-
resent a fairly well denned category of sense organs
which, if not so sharply marked off as those of taste and
of smell, are fairly comparable in distinctness with the
receptors for heat, cold, or pain. The extent of their
occurrence is easily recognized. Thus the vapor of
ammonia not only irritates the nose, but also the eye,
causing watering, as well as the mouth and the upper
respiratory region whence arise impulses that lead to
coughing and choking. Irnlnnts of this kind also stim-
ulate the anus and the genital apertures and in fact any
THE COMMON CHEMICAL SENSE 103
part of the body where a mucous surface is in contact
more or less with the exterior. In man, then, the recep-
tors for irritants have a much wider distribution over
the body than the olfactory receptors have in that they
are found on almost every exposed or partly exposed
2. In Lower Vertebrates. In other mammals than
man, in birds, and in reptiles the receptors for irritating
substances are probably distributed in much the same
way as in man and are confined to the exposed or semi-
exposed mucous surfaces. In the amphibians and the
fishes, however, this system of receptors shows a pro-
digious expansion in that in these animals it is found
covering their whole exteriors. The well known experi-
ment of stimulating the frog's foot with solutions of
acids and other such substances is based upon this peculi-
arity and the sensitiveness of the skin of this and other
amphibians and of fishes as worked out by Nagel (1894),
Parker (1908a, 1908b, 1912), Sheldon (1909), Cole (1910),
Crozier (1915, 1916), and others show quite clearly that
sensitiveness to solutions of chemicals is a common
property of the skin in all these aquatic vertebrates.
As early as 1894 Nagel discovered that the integument
of the dogfish Scyllium was extremely sensitive to a great
variety of chemical substances. He likewise found that the
skin of the goosefish Lophius and of the lancetfish Amphi-
oxus were also generally open to chemical stimulation.
Nagel 's observations on Amphioxus were confirmed
in 1908 when it was shown that the skin of this fish was
sensitive to solutions of acids, alkalis, alcohol, ether,
chloroform, turpentine, oil of bergamot and oil of rose-
mary but not to solutions of sugar. It was also demon-
104 SMELL, TASTE, ALLIED SENSES
strated that the skin of the catfish Amiurus was sensitive
to sour, saline, and alkaline solutions, a condition that
was subsequently found to be true for the young of the
lamprey eel Ammocoetes Parker (1908b, 1912). In 1909
Sheldon published an account of the chemical stimulation
of the skin of the dogfish Mustelus, the most extensive
study of this kind thus far made. Sheldon found that
the whole outer surface of this fish was very sensitive
to acids and alkalis, less so to salts and bitter substances
and not at all to sugar solutions, a condition that in gen-
eral confirmed the results of earlier workers. Crozier
(1915) studied the mutual relations of salts of sodium,
potassium, and calcium as applied to the frog's skin and
was able to demonstrate ionic antagonism which led him
to conclude that in normal stimulation the surface of the
receptor must be penetrated by the stimulant.
These observations warrant the general conclusion
that the outer surfaces of most fishes and amphibians are
open to stimulation by chemical substances of a mildly
irritating kind. It is probable that this capacity has
been retained by the air-inhabiting vertebrates in only
a very circumscribed and local way, namely on those
exposed or partly exposed mucous surfaces that reproduce
in their delicacy and moistness the characteristics of the
general outer surface of aquatic forms. From this
standpoint the restriction of the chemical sensibility of
the air-inhabiting vertebrates is the result of the drying
of their skins in consequence of an ancestral migration
from an environment of water to one of air.
3. Nerve Terminals. The form of nerve terminal
that is concerned with the reception of chemical irritants
in the skin of vertebrates is well indicated in the catfish
THE COMMON CHEMICAL SENSE 105
Amiurus. If a bait in the form of a piece of meat or the
like is held close to the flank of one of these fishes, the
animal is very likely to turn suddenly and snap it up. This
is not a surprising response, for the sides of these ani-
mals are well provided with taste-buds. They are also
supplied with lateral-line organs. Both these sets of
receptors may be eliminated by cutting on the one hand,
the branch of the facial nerve that is supplied to the taste-
buds of the side of the body and, on the other, the lateral-
line nerve that is distributed to the lateral-line organs
of the same region. After the fish has recovered from
such an operation, it will no longer respond to a bait held
near its flank, but the skin of this region is still per-
fectly open to stimulation by sour, saline and alkaline
solutions. As the only receptors left after the operation
just described are the free-nerve terminals of the spinal
nerves, these terminals must be the receptors for chem-
ical irritants. This conclusion is in accord with the fact
that this type of ending is the only one that occurs in
many portions of the skin of the dogfish, of the foot of
the frog, and of the partly exposed mucous surfaces of
the higher vertebrates such as those of the mouth and
the nose. Moreover when these endings are rendered
inoperative by cutting their nerve trunks, as Sheldon did
on the dogfish and as has often been done on the nasal
cavities of mammals, irritating substances are no longer
effective. Free-nerve endings of spinal or cranial nerves
are, therefore, quite certainly the type of nerve-terminal
concerned with the reception of chemical irritants.
4. Relation to Other Senses. In discussing the relation
of the receptors for chemical irritants to other sense
organs some of the earlier workers suggested a compari-
106 SMELL, TASTE, ALLIED SENSES
son of these receptors with those for taste Parker
(1908a); Herrick (1908). More recently Cogliill (1914)
has declared that since tactile and chemical irritability
develop simultaneously in certain amphibian larvae,
chemical irritability is in reality tactile in nature. It
must also be perfectly evident that the receptors under
consideration have striking resemblances to those con-
cerned with pain.
The fact that organs of taste always involve special-
ized end-organs, such as taste-buds, whereas receptive
surfaces for chemical irritants may contain only free-
nerve endings, shows that the relation of these two
classes of receptors is at best only distant. This con-
clusion is supported by an observation by Parker and
Stabler (1913) that the minimum concentration of ethyl
alcohol necessary for the stimulation of the irritant
receptors in man, 5 to 10 molar, is decidedly stronger
than that which will stimulate the human gustatory
organs, 3 molar.
The relation of the receptors for irritants to those
for touch and for pain seems to be clearly indicated in
the results of experiments in which exhaustion and nar-
cotics have been used. If the tail of an amphioxus is
subjected to about twenty applications of weak nitric
acid, 0.025 molar, in fairly rapid succession, the fish will
cease to respond to this kind of stimulus. After the
exhaustion of the mechanism for this type of reception,
the tail of the fish will be found fully sensitive to the
touch of a camel's hair brush. If, now, the tail of
a fresh individual is vigorously stroked some thirty times
in succession, the fish will cense to respond to this form
of mechanical stimulation but it will still be found very
THE COMMON CHEMICAL SENSE 107
sensitive in the exhausted part of the skin to weak acid.
Thus mechanical stimulation and chemical stimulation
seem to apply to different sets of terminals and the
exhaustion of one set does not involve that of the other.
On treating a portion of the surface of a dogfish with
2 per cent cocaine, Sheldon found that tactile stimulation
ceased in from ten to twenty minutes whereas chemical
stimulation was effective for a somewhat longer period.
By continuing the treatment with cocaine receptivity for
chemical irritants was also eventually abolished. In a
similar way Cole (1910) found that if the hind foot of a
spinal frog was treated with 1 per cent cocaine till the
animal no longer responded to pricking or scratching
with a needle or to pinching with forceps, it would never-
theless respond vigorously to a salt solution. The'se
results were confirmed by Crozier in 1916 who used a
half per cent solution of cocaine hydrochloride on a
frog's foot. After about 20 minutes' immersion in this
solution, the reaction time of the cocained foot to formic
acid 0.05 molar, was about twice that of the normal foot.
After about an hour to an hour and a half of this treat-
ment the cocained foot no longer reacted to pinching but
gave good responses to acid with reaction times of from
ten to fifteen seconds, about twice that of the non-cocained
foot. These observations show beyond a doubt that the
effect of chemical irritants on the naturally moist skin
of vertebrates is not to be ascribed to the stimulation
of organs of touch or of pain but to some other form of
receptor, the terminals of what has been called the com-
mon chemical sense.
As Crozier has pointed out, there can be no question
of the distinctness of the human sensations attributed
108 SMELL, TASTE, ALLIED SENSES
to the common chemical sense as contrasted with our
sensations of smell, taste, touch, or pain. The curious
feeling that comes from vapors that irritate the eyes,
nose, or even the mouth has not the remotest relation
to touch, smell, or taste and is only distantly suggestive
of pain. Pain, however, is easily separated from the
common chemical sense by the use of cocaine, and we
are, therefore, entirely justified in concluding that the
common chemical sense is a true sense with an indepen-
dent set of receptors and a sensation quality entirely its
own. In the fishes and amphibians it pervades the whole
integument but in the reptiles, birds and mammals it is
restricted to the partly exposed mucous membranes of the
natural apertures, a restriction that doubtless arose as
the vertebrate changed from an aquatic to an air-inhabit-
BBAEUNIN T G, H. 1904. Zur Kennitnisa der Wirkung chemischer Reize.
Arch. ges. Physiol., Bd. 102, pp. 163-184.
COGHILL, G. E. 1914. Correlated Anatomical and Physiological Studies
of the Growth of the Nervous System of Amphibia, I. The Afferent
System of the trunk of Amblystoma. Jour. Comp. Neurol., vol. 24,
pp. 161-233. 1919. II. The Afferent System of the head of Ambly-
stoma. Jour. Comp. Neurol., vol. 26, pp. 247-340.
COLE, L. \V. 1910. Reactions of Frogs to Chlorides of Ammonium, Potas-
sium, Sodium, and Lithium. Jour. Comp. Neurol. Psychol., vol. 20,
CROZIEB, W. J. 1915. Ionic Antagonism in sensory Stimulation. Amer.
Jour. Physiol, vol. 39, pp. 297-302.
CROZIKR, W. J. 1916. Regarding* the Existence of the " Common Chemical
Sense " in Vertebrates. Jmir. Comp. Neurol., vol. 26, pp. 1-8.
HERRICK, C. J. 1908. On the phylogenetic Differentiation of the Organs
of Smell and Taste. Jour. Comp. Neurol. Psycho*., vol. 18, pp. 159-166.
LOEB, J. 1905. On the Production and Suppression of Muscular Twitch-
ings arid Hypersensitiveness of the skin by Electrolytes. Studies in
<:neral Physiology, vol. 2, pp. 748-765.
THE COMMON CHEMICAL SENSE 109
NAGEL, W. 1894. Vergleichend physiologische und anatomische Unter-
suchungen iiber den Geruchs- und Geschmackssinn und ihre Organe.
Bibl. Zool., Heft 18.
PARKER, G. H. 1908a. The Sense of Taste in Fishes. Science, vol. 27,
PARKER, G. H. 1908b. The Sensory Reactions of Amphioxus. Proc.
Amer. Acad. Arts. Sci., vol. 53, pp. 415-455.
PARKER, G. H. 1912. The Relation of Smell, Taste, and the Common
Chemical Sense in Vertebrates. Jour. Acad. Nat. Sci. Philadelphia,
vol. 15, pp. 221-234.
PARKER, G. H. and E. M. STABLER. 1913. On Certain Distinctions be-
tween Taste and Smell. Amer. Jour. Physiol., vol. 32, pp. 230-240.
SHELDON, R. E. 1909. The Reactions of the Dogfish to Chemical Stimuli.
Jour. Comp. Neurol. Physchol., vol. 19, pp. 273-311.
ANATOMY OF THE GUSTATORY ORGAN.
Contents. 1. Distribution of Taste-buds in the Oral
Cavity of Man. 2. Comparative Distribution of Taste-
buds. 3. General Form of Taste-buds. 4. Cellular Com-
position of Taste-buds. 5. Intragemmal and Other
Spaces. 6. Innervation of Taste-buds. 7. Gustatory
Nerves. 8. Eelation of Gustatory Nerve Fibers and
Taste-buds. 9. Bibliography.
1. DISTRIBUTION of Taste-buds in the Oral Cavity of
Man. In man the organs of taste are located in the
mouth. These are the so-called taste-buds discovered
independently by Loven (1867) and by Schwalbe (1867).
In the adult human being they have been identified on
the dorsal surface of the tongue except the mid-dorsal
region, on both the anterior and posterior surfaces of the
epiglottis, on the inner surface of the arytenoid process
of the larynx, on the soft palate above the uvula, on the
anterior pillars of the fauces, and on the posterior wall
of the pharynx. All other oral surface in the adult, such
as the lips, the gums, the cheeks, the inferior surface of
the tongue, the hard palate, the uvula, and the tonsils
are devoid of these organs.
In young individuals, babes, and human embryos
taste-buds are more widely distributed than they are in
the adult. According to Tuckerman (1890a, 1890b) and
Graberg (1898) taste-buds appear in man at about the
beginning of the third month of foetal life. Stahr (1902)
found them in human embryos in the middle of the dor-
ANATOMY OF THE GUSTATORY ORGAN 111
sum of the tongue and Ponzo (1905) identified them on
the palatine tonsils, the hard palate, and the cervical
part of the esophagus, regions from which they are
absent in the adult. As early as 1875 Hoffmann called
attention to the fact that in human embryos and newly
born babes taste-buds were commonly found on the free
surfaces of the vallate papillae, situations from which
they disappear in later life. This observation was con-
firmed by Tuckerman (1889) as well as by Hermann
(1885), who, however, worked upon the rabbit. Thus the
gustatory apparatus of man and of other mammals is
by no means constant, but suffers reduction from the late
embryonic period to the adult state. On the tongue of
man the reduction is chiefly in the middle region of the
distal two-thirds so that, as Stahr (1902) has pointed
out, the center of taste in this organ shifts with growth
from a position near the tip of the tongue to one in the
neighborhood of the vallate papillae. This opinion is in
agreement with the observation of Heiderich (1906) that
after birth the taste-buds of the vallate papillae show
almost no change.
Wherever taste-buds occur in man, except on the
tongue, they are found simply imbedded in the epithe-
lium of the mucous membrane of the region concerned.
On the tongue, however, they are almost invariably asso-
ciated with certain kinds of papillae. The human tongue
possesses several classes of these structures, which from
their forms have been designated as conical, filiform,
fungiform, foliate, and vallate. The plush surface of the
dorsum of the tongue is produced by innumerable fine
conical and filiform papillae. These, however, almost
never have taste-buds associated with them. The other
SMELL, TASTE, ALLIED SENSES
types of papillae, the fungiform, foliate, and vallate, very
generally carry taste-buds (Fig. 25).
The fungiform papillae are relatively large knob-like
elevations scattered over the dorsum of the tongue.
They can be easily seen with the unaided eye and may
be readily located and identified. They commonly carry
a few taste-buds embed-
ded in the epithelium of
their free outer sur-
faces. In sections of the
crowns of these papillae
parallel to the surface
of the tongue three or
four or more, rarely six
to eight, taste-buds may
be identified. In verti-
cal section it can be
seen that the taste-buds
are not indiscriminately
scattered over the free
surface of the papilla,
but are perched on the
secondary dermal pa-
pillae contained within the papilla proper and that they
always reach through the full thickness of the epidermis
from the dermal core of the secondary papilla to the free
outer surface of the primary papilla itself (Fig. 26).
This extension through the whole thickness of the epi-
dermis seems to be a common characteristic of taste-
buds, for it is to be noted in them from fishes to man.
It is an easy means of distinguishing them from other
bud-like receptors such as the lateral-line organs whose
Fio. 25. Dorsal view of the human tongue
ehowing foliate papillw (f) and vallate papilla (v).
ANATOMY OF THE GUSTATORY ORGAN 113
cells extend only part way through the epithelium in
which they are imbedded.
The foliate papillae lie on either side of the edge of
the human tongue and close to its root. They form a
series of from three to eight vertical parallel ridges.
Each ridge is abundantly supplied with taste-buds which,
Fio. 26. Vertical section of a fungiform papilla showing two taste-buda.
however, do not occur on its free outer surface but on
its sides. Here the buds open into the ditch between the
ridge on which they are located and the next one. In
sections transverse to the axis of the ridge the numbers
of taste-buds seen on the two sides of a given ditch may
vary from three to twenty. In the rabbit the foliate pa-
pillae are especially well developed and are abundantly
supplied with taste-buds. These have been very fully
studied recently by Heidenhain (1914) who has shown
that the buds are arranged in more or less vertical rows
on each papillar fold and that they probably increase
in numbers by a process of fission.
The vallate papillae, which in man are usually six to
twelve in number, form on the posterior part of the
114 SMELL, TASTE, ALLIED SENSES
tongue a V-shaped row whose angle points toward the
esophagus (See Fig. 25). Each papilla is a low circu-
lar elevation surrounded by a relatively deep, narrow
ditch. The taste-buds are located on the walls of this
ditch and chiefly on that wall which forms part of the
papilla. In a vertical section through a vallate papilla,
it is usual to see on the side of the ditch formed by the
FIG. 27. Vertical section of a vallate papilla showing taste-buds.
papilla from ten to a dozen taste-buds and on the side
away from that structure four to six such bodies
(Pig. 27). However, as Schwalbe (1868) long ago
pointed out, much individual variation occurs and it is,
therefore, very difficult even to estimate with any degree
of accuracy the total number of taste-buds on a single
papilla. W. Krause (1876) believed the number for a
single papilla in man to be as high as 2500, but von Wyss
(1870) placed it much lower, namely, at about 400. Even
these figures seemed too high to Graberg (1899) who gave
the maximum at 100 to 150 and the minimum at 40 to 50.
Heiderich (1906) made a close count on 92 papillae .from
human beings ranging in age from the first to the twen-
tieth year and found the extreme numbers of buds to a
ANATOMY OF THE GUSTATORY ORGAN 115
papilla to be 508 and 33 with an average not far
2. Comparative Distribution -of Taste-buds. Taste-
buds, like the olfactory receptors, require a moist sur-
face. It is, therefore, not surprising to find that in all
air-inhabiting vertebrates they are limited to the oral
cavity. Their distribution in mammals has been very
fully studied by Tuckerman (1892), Munch (1896), and
Taste-buds also appear to be limited to the oral
region in amphibians notwithstanding the fact that many
of these animals possess a permanently moist skin. In
fishes they were apparently first seen by Leydig in 1851
and were subsequently described by Schulze (1863). In
these forms they are not restricted to the oral region.
According to Johnston (1906) they are present on the
heads of cyclostomes as well as on those of ganoids where
they were studied by Dogiel (1897). Herrick (1918)
states that in some bony fishes, such as the catfishes, the
carps, and the suckers they are to be found over the
entire outer surface of the body and this investigator
(1903) has further shown that in the catfish Amiurus the
taste-buds on the flank of the fish are as significant in the
detection of bait as are those about the mouth (Fig. 28).
3. General Form of Taste-buds. Taste-buds vary in
form from that of a flask to that of a. spindle. Commonly
they are single bud-shaped bodies opening to the exte-
rior by a small pore (Fig. 29). Compound buds in which
the body of the bud appears double and two pores are
present have long been known and Heidenhain (1914) has
recently shown that this condition may reach an extreme
degree of complexity in the foliate papillae of the rabbit
116 SMELL, TASTE, ALLIED SENSES
where compound buds with as many as six pores have
been identified. The frequency with which types of buds
with different numbers of pores occur may be gathered
from the enumeration by Heidenhain who found that in
509 taste-buds from the foliate papillfe of the rabbit
368 had one pore, 100 two pores, 29 three pores, 7 four
Fio. 28. Lateral view of the catfish, Amiurus melas, showing in black the gustatory
branches of the facial nerve. After Herrick, 1903, Fig. 3.
pores, 1 five pores, and 4 six pores. In the compound
buds the pores usually form a more or less linear series
and as each pore represents a single element in the com-
plex the whole gives the impression of a row of fused
buds (Fig. 30.) These compound buds are believed to
result from a process of imperfect division.
Some taste-buds open directly on the oral surface
where they are located ; others are marked by a pore, the
outer taste-pore, which leads into a short canal and this
in turn ends at the inner taste-pore formed by the distal
end of the bud itself. Von Ebner (1897) noted that in
some instances the canal expanded into a small chamber
or ampulla over the tip of the bud and, though Grabcrg
(1899) could not. confirm this statement for man, the
condition has been observed anew by Kallius (1905) in
ANATOMY OF THE GUSTATORY ORGAN 117
human material and by Heidenhain (1914) in the rabbit.
5. Cellular Composition of Taste-buds. The cells
composing the taste-buds are so arranged as to give each
bud somewhat the appearance of a flower bud or of a
leaf bud not yet unfolded. As has been stated already,
these end-organs were described in the skin of fishes as
early as 1851 by Leydig and were
subsequently simultaneously and
independently discovered in the
mouths of the higher vertebrates
<by Loven (1867) and by Schwalbe
(1867). The older workers
usually distinguished in the taste-
buds two classes of cells, taste-
cells, which were supposed to be
chiefly central in position, and
supporting cells mainly on the
exterior of the bud.
Each taste-cell is an attenuated delicate structure
whose elongate nucleus forms a slight enlargement near
the middle of the cell-body (See Fig. 31a). Distal to
it narrows to a delicate process, the taste hair. This
hair either projects out of the pore into the exterior or
into the canal when that is present. Proximal to the
nucleus the taste-cell extends into the deeper part of the
bud there to terminate usually in a small rounded knob.
The number of taste-cells in a bud varies from two or
three to as many as the contained supporting cells, per-
haps ten or more.
Beside the taste-cells proper Schwalbe (1867) de-
scribed what he believed to be a second form of receptive
Fio. 29. A simple taste-bud
from a foliate papilla of the
rabbit. After Heidenhain, 1914,
Plate 19, Fig. 5.
SMELL, TASTE, ALLIED SENSES
Fio. 30. A compound] taste-
bud from a foliate papilla of the
rabbit. After Heidenhain 1914, Plate
23, Fig. 27.
cell to which he gave the name of "Stabzelle" or rod cell.
This type of cell was said to differ from the ordinary
taste cell in that it was without a taste hair. It has not been
identified with certainty by subsequent investigators.
The supporting cells of the taste-buds have been the
occasion of much difference of opinion. The older
workers believed that these
cells were limited to the exte-
rior of the buds, but Merkel
(1880) showed that they also
occurred in the interior and
Eanvier (1888) . definitely
described both inner and outer
supporting cells. Hermann
(1889) concluded that these
two classes of supporting cells
differed not only in position but also in structure.
The outer cells, which he called pier cells (Pfeilerzellen),
were relatively large pyramidal elements whose
nuclei were proximal in location and whose distal
ends terminated in a zone marked with fine vertical
stripings. For the inner supporting cells Hermann used
Schwalbe's term of rod cells (Stabzellen) without, how-
ever, wishing thereby to imply that they were of a sensory
nature. They were said to differ from the pier cells
in that they were devoid of the peripheral striped zone.
Hermann also described basal supporting cells which to
the number of two to four were found in the proximal
part of the taste-buds. Von Lenhossek (1893b) doubted
the existence of basal cells and described four not very
sharply separate types of supporting cells. Graberg
(1899) reidentified in human material the basal cells dis-
ANATOMY OF THE GUSTATORY ORGAN 119
covered by Hermann. The other supporting cells were
described by this author as either central or peripheral
and were to be distinguished from each other rather by
location than by differences of structure.
The indefiniteness and uncertainty that surrounded
the question of the classes of supporting cells in taste-
buds has been dissipated in large part by the declaration
of Kolmer (1910) that between the taste-cells on one
hand and the so-called supporting cells on the other there
are all possible transitions and that it is, therefore, a
mistake to attempt to draw distinctions not only between
various kinds of supporting cells but between supporting
cells and taste-cells. Kolmer believed that all the elon-
gated cells in taste-buds are really taste-cells and that
their differences are due to the stage of growth at which
they are for the moment. This opinion, which is sup-
ported by what is known of the innervation of the taste-
buds, has gained the acceptance of the more important
recent workers in this field, such as Retzius (1912) and
Heidenhain (1914). If true, it shows the taste-bud to be
a much more unified structure than has heretofore been
supposed and it does away at once with the confusion
over the classes of cells that were believed to enter into
The basal cells apparently do not fall into this general
category of more or less differentiated receptor cells,
but, according to Heidenhain at least, they are elements
that only under certain conditions are regularly present
and are concerned with the division of the buds.
The epidermal cells immediately next the taste-bud
are often flattened against this structure and conform
more or less to its outline. These cells have been called
120 SMELL, TASTE, ALLIED SENSES
by Grabcrg (1899) extrabulbar cells and though they are
not to be classed as part of the bulb proper they are
nevertheless sufficiently related to that structure to be
appropriately mentioned in this connection.
As Hermann (1889) long ago pointed out, the cells of
the taste-buds are probably undergoing continual change.
Old cells are degenerating and disappearing and new
ones are forming to take the places of those that have
broken down. The degenerating process is indicated by
the presence in the taste-buds of cells in all stages of
growth and of considerable numbers of leucocytes, as
pointed out by Ranvier (1888), von Lenhossek (1893b),
and others. The regenerative process is shown in the
occasional occurrence of mitotic figures in the base of
the bud thus giving evidence of cell division in that region
5. Intragemmal and other Spaces. Graberg (1899)
has called attention to the fact that taste-buds are not
solid structures but that their cells are separated one from
another by considerable intervening space, and that
much free space occurs in the tissue immediately around
the buds. This intra-, peri-, and subgemmal space is be-
lieved by Graberg not to be an artifact, for it can be
identified by almost all methods of preparation. Accord-
ing to this investigator these various spaces communicate
with one another and connect with the exterior through
the taste pore. They may be the means of irrigating and
thereby cleaning the taste-bud, for it is possible that
fluid may flow slowly through them from the interior to
6. Innervation of Taste-buds. Among the older in-
vestigators the innervation of the taste-buds was a ques-
ANATOMY OF THE GUSTATORY ORGAN 121
tion of much uncertainty. Some claimed that the
gustatory nerve-fibers connected directly with the cells
of the taste-buds; others that they did not so connect.
The first to employ special neurological methods for the
solution of this question were Fusari and Panasci (1890).
These workers claimed that by means of Golgi prep-
FIG. 31. Golgi preparations of the taste-buds of the rabbit, a showing cells (after
von Lenhosse'k, 1893a, Fig. la) and 6 showing nerve-terminations (after Retzius, 1892a,
PlateS, Fig. 4).
arations it could be shown that the gustatory cells were
directly connected with nerve-fibers. Two years later
Retzius (1892a) published an account of the innervation of
the taste-buds of mammals and of amphibians in which
he showed in preparations stained by methylenblue as
well as by the Golgi process that the nerve-fibers were
not directly connected with the taste-cells but ended in
close proximity to them (Fig. 31). These results were
confirmed in 189 3 by von Lenhossek, Arnstein, and Jacques
as well as by the subsequent work of Retzius himself
(1893) and there seems to be no ground for doubting
the correctness of the general conclusion arrived at more
or less independently by these four investigators.
The anatomical relations shown by these workers
are relatively simple. From the subepithelial nerve
plexus in the neighborhood of taste-buds fibers pass out-
ward into the epidermis. These fibers either form sys-
SMELL, TASTE, ALLIED SENSES
terns of branches ending in free terminations around a
taste-bud, in which case they are called perigemmal or
Fio. 32. Golgi preparations of the taste-buds of the cat, a, in longitudinal section
fihowing nerve terminations, and 6, in transverse section showing intrageinmal nerve
ebers. After Retzius, 1892a, Plate 7, Figs. 1 and 4.
peribulbar fibers, or they enter the bud and end freely
among its cells being designated then as intragemmal
or intrabulbar fibers (Fig. 32). As the figures given by
Retzius, Arnstein, and others show, the nerve-fibers in
ANATOMY OF THE GUSTATORY ORGAN 123
the buds are as intimately applied to the so-called sup-
porting cells as to the taste-cells, showing, as has al-
ready been stated, that the distinction between what
has been assumed to be two classes of cells is probably
In addition to intergemmal and iperigemmal fibers,
which in consequence of their close relations with the
taste-buds may be designated as gemmal or bulbar fibers,
there are also fibers that pass into the undifferentiated
epithelium between the buds and end close to the external
surface as free-nerve terminations. These have been
called intergemmal fibers, but it is doubtful whether
they have anything to do with taste and it is not improb-
able that they are concerned with other sensory functions
such as the common chemical sense, pain, and the like, in
which case a designation implying relations to a taste-bud
is in no sense appropriate.
Taste-buds such as have already been described have
been found in a wide range of vertebrates. They
not only occur in mammals, where their relation with
the nerve-fibers was first correctly described by Retzius
( 1892a) , but also in fishes as seen by Retzius ( 1892a, 1893 ) ,
vonLenhossek (1893a),Dogiel (1897) and others (Fig. 33).
It is, therefore, probable that so far as essentials are con-
cerned the innervation of the taste-buds of all vertebrates
presents a relatively uniform plan.
7. Gustatory Nerves. There are no separate gusta-
tory nerves in the vertebrates as there are olfactory nerves
or optic nerves. Gustatory fibers occur in several crani-
al nerves and it is by means of these that the taste-buds
of various regions are provided with those nervous con-
nections that have been described in the preceding section.
SMELL, TASTE, ALLIED SENSES
In the fishes the nerves chiefly concerned are the vagus,
the glossopharyngeal and the facial. The taste-buds of
the gill region are supplied by the vagus and the glosso-
pharyngeal. Those that are in the mouth proper or are
on the exterior of the body are innervated by the facial
ni 1-ve. Consequently in the catfish (See Fig. 28), in
which the whole outer
skin is provided with
taste-buds, this nerve is
enormously developed and
sends large branches to
the barbels and an exten-
sive recurrent branch to
the flanks of the body
In mammals, includ-
Hlg mail, tllC llinerVatlOll
of the taste-buds is not
upon so simple a plan as in fishes. In these higher
vertebrates gustatory fibers may possibly be contained
in four of the cranial nerves, the vagus, the glosso-
pharyngeal, the facial, and the trigeminal. The
distribution of these nerves in the human tongue has been
worked out by Zander (1897). Certain parts of the
vagus are distributed to the larynx and to the epiglottis
as well as to the most posterior part of the tongue itself
and innervate very probably the taste-buds of these re-
gions (Fig. 34). The glossophaiyngeal supplies the pos-
terior third of the tongue including the foliate and vallate
papillae, for, as was first shown by von Vintschgau and
Honigsf.hmied (1876), when the ninth nerve is cut the
taste-buds of these parts soon degenerate and disappear.
Although the correctness of this observation was denied
Fio. 33. Golgi preparations of the taste-
buds of the common European barbel show-
ing cells and nerve-fibers. After von Lenhossek,
1893 a . Fig. 2.
ANATOMY OF THE GUSTATORY ORGAN
by Baginsky (1894), it has been confirmed by such a
number of observers, including Drasch (1887), Ranvier
(1888), Sandmeyer (1895), Meyer (1897) and others, that
it is now generally accepted. Both the right and the
left branches of this nerve innervate the median vallate
papilla in mammals and form at the base of this organ,
as Vastarini-Cresi (1915) has
shown, more or less of a gusta-
tory chiasma. The anterior two-
thirds of the tongue in man are
innervated by the lingual nerve
which is made up of a union of
the lingual branch of the trigem-
inal nerve with the chorda
tympani of the facial. It has
been an open question whether
the gustatory fibers for this part
of the tongue belong to the
trigeminal, to the facial, or
possibly even to the glosso-
pharyngeal, for all these nerves
intercommunicate through a
plexus of fine branches near their roots. F. Krause (1895)
noted the effect on taste of the complete extirpation of the
ganglion of the trigeminal nerve, and found that in some
instances taste was entirely obliterated from the appro-
priate part of the tongue, but that in others it was only
somewhat reduced. These differences do not appear in
the later and more conclusive work of Gushing (1903)
who found that, when time enough was given, all subjects
from whom the ganglion of the trigeminal nerve had
been removed, recovered taste completely. He attributed
FIG. 34. Diagram of the
human tongue showing the parts
innervated by the lingual nerve
(horizontal lines), by the glosso
pharyngeal nerve (oblique lines),
and by the vagus nerve (small
circles). After Zander, 1897.
SMELL, TASTE, ALLIED SENSES
the temporary disturbance in taste, a condition that was
supposed to be permanent by Krause, to the effect of the
degenerating trigeminal fibers on the adjacent gusta-
tory fibers, an effect that disappeared when the degenera-
tion was complete. Consequently Gushing concluded that
the gustatory fibers from the anterior part of the tongue
FIG. 35. Diagram to illustrate the possible paths of the gustatory nerve-fibers from the
tongue to the brain in man. The distal part of the tongue (1) is innervated by the lingual
nerve (2) whose gustatory fibers pass to the brain by way of the chorda tympani (3), a
branch of the facial nerve (VII). The proximal part of the tongue is innervated^ by the
glossopharyngeal nerve (4). The undoubted gustatory paths over the facial nerve (VII) and
the glossopharyngeal nerve (IX) are indicated by dotted lines. The commonly assumed
Eaths by way of the trigeminal nerve (V) are shown in heavy black lines with arrows. Modi-
ed from Cushing, 1903.
are not part of the trigeminal nerve. If this is so, they
must belong to the facial or possibly to the glossopharyn-
geal nerve (Fig. 35). That they are abundantly present
in the chorda tympani of the facial nerve is well known
from the fact that direct stimulation of the chorda in
the neighborhood of the ear drum is commonly accom-
panied by sensations of taste, but whether these gusta-
tory fibers on reaching the facial nerve pass into the brain
through its root or make their way to the root of the glos-
sopharyngeal is not yet definitely settled. It is, therefore,
probable that in mammals the trigeminal nerve, though
ANATOMY OF THE GUSTATORY ORGAN 127
suspected of including gustatory fibers, is really devoid
of them. These fibers at most occur in the facial, glos-
sopharyngeal and vagus nerves, but none of these nerves
is exclusively gustatory.
8. Eelation of Gustatory Nerve-fibers to Taste-buds.
It is an interesting and significant fact that on the de-
generation of the gustatory nerve-fibers the taste-buds
associated with them should disappear. This state of
affairs, long ago demonstrated for mammals, has recently
been shown by Olmsted (1920a, 1920b) to occur also in
fishes. Meyer (1897) showed that thirty hours after cut-
ting the glossopharyngeal nerve in the rabbit the taste-
buds began to show a change and that by the end of seven
days most of them had disappeared. In the catfish Ami-
urus, according to Olmsted, the taste-buds on the oral
barbels begin to degenerate in a little over ten days after
the nerve to these organs has been cut and they com-
pletely disappear by the end of the thirteenth day.
Ranvier (1888) believed that in mammals the taste-buds
were destroyed by wandering cells, but Sandmeyer (1895)
and Meyer (1897) held the view that the gustatory cells
suffered dedifferentiation and changed into ordinary epi-
thelial cells. In Amiurus Olmsted has found strong
evidence in favor of the destruction of the cells of the
taste-buds by phagocytes thus supporting Ranvier 's
Olmsted has shown, further, that on the regeneration
of a nerve in a denervated Amiurus barbel from which
all the taste-buds had disappeared, new buds reappear
coincident with the arrival of the nerve. With the de-
generation of the nerve and the loss of the taste-buds the
barbels lose their receptivity for sapid materials, nor
128 SMELL, TASTE, ALLIED SENSES
does this return till the buds regenerate. Since the taste-
buds disintegrate with the loss of the nerve and new ones
form only with the regeneration of this structure, it is
clear that the bud is dependent upon the nerve. As
Olmsted has suggested, it is probable that when a twig of
the nerve reaches a given spot in the epidermis, it gives
out a substance, hormone-like in character, that excites
the epithelial cells of that spot to form a bud much as the
embryonic eye cup of the vertebrate excites in the super-
imposed ectoderm the formation of a lens. In this way at
least the intimate dependence of the taste-bud on the re-
generating nerve-fiber can be explained and,, judging
from the account given by Landacre (1907) of the ontoge-
ny of these organs, a similar explanation may also
apply in development.
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PHYSIOLOGY OF GUSTATION.
Contents. 1. Location of Taste. 2. Gustatory stimulus.
3. Qualities of Taste. 4. The Sour Taste. 5. The Saline
Taste. 6. The Bitter Taste. 7. The Sweet Taste. 8.
Inadequate Stimuli. 9. Distribution of Tastes on the
Tongue. 10. Action of Drugs on Taste. 11. Substances
with two Tastes. 12. Latency of Taste Sensations. 13.
Taste Alterations ; After-tastes. 14. Gustatory Contrasts.
15. Taste Compensations and Mixtures. 16. The Gusta-
tory Senses. 17. Comparative. 18. Bibliography.
1. LOCATION of Taste. Although in man taste is not
strictly limited to the mouth, for it spreads into some
of the adjacent cavities, it is primarily located in the
buccal space and is especially a function of the tongue.
When the mouth of a normal adult is explored witli solu-
tions of sapid substances, many parts such as the lips,
the gums, the floor, the lower surface of the tongue, the
inner surfaces of the cheeks, and the hard palate are
found to be insensitive to taste. Even the uvula which,
according to many of the older workers, was regarded as
having a gustatory function, has been shown by Kiesow
and Hahn (1901) not to be concerned with taste. All
these regions are well known to be devoid of taste-buds.
Whether the pillars of the fauces and the tonsils have
to do with taste is a matter of dispute. Hanig (1901)
believed that these parts have a gustatory function, but
Kiesow and Hahn (1901) regarded them as usually in-
sensitive. The mucous membranes of the following parts
PHYSIOLOGY OF GUSTATION 133
are concerned with taste; the beginning of the gullet,
the region of the arytenoid cartilages within the larynx,
the epiglottis, the soft palate, and particularly the tongue.
In all these regions taste-buds have been identified. On
the tongue of adult human beings taste is limited to the
tip, the lateral margins and the dorsal surface of the
root, the large central area on the upper surface of this
organ being devoid of taste. In children, as contrasted
with adults, the whole upper surface of the tongue in-
cluding the central area is said to be sensitive to taste
as is also the inner surfaces of the cheeks.
2. Gustatory stimuli. The stimulus for taste is an
aqueous solution of a great variety of substances. Mate-
rials insoluble in water are tasteless,, but not all substances
that form aqueous solutions have taste. Thus oxygen,
hydrogen, and nitrogen, though freely soluble in water,
are without taste. Piutti (1886) long ago showed that
Isevo-asparagine is tasteless, although its stereoisomer
dextro-asparagine is sweet. Other organic compounds,
such as the carbohydrates raffinose and alpha-galaoctite
are said to be almost, if not quite, tasteless.
When solids or semi-solids are chewed in the mouth,
they not only become mixed with the saliva whereby many
of their components become dissolved, but they are
spread over the surface of the tongue and are thus
brought into intimate contact with its taste-buds. In fact
it is not improbable that the movement of the tongue fa-
cilitates the entrance of these solutions into the pores of
the taste-buds. At least solutions placed upon the
tongue, particularly near its root, are tasted with greater
certainty, when this organ is moved about than when it
is held still.
134 SMELL, TASTE, ALLIED SENSES
3. Qualities of Taste. Tastes, unlike odors, fall into
a limited number of well-circumscribed groups, which
have received distinctive names such as sour, saline, bit-
ter, sweet, and the like. The multitude of flavors and
other sensations associated with our food are undoubt-
edly mixed in character and include touch, heat, cold, the
common chemical sensation, and especially odor. By ap-
plying materials in weak solution, at the temperature of
the mouth and with the nostrils closed, extraneous sen-
sations may be eliminated and there remains a certain
irreducible residue, the tastes. Zenneck (1839), Valentin
(1848), Duval (1872) and later Sternberg (1898) admit-
ted only two classes of tastes, sweet and bitter. Stich
(1857), however, long ago showed that sour was a sensa-
tion produced by stimulating only a limited part of the
buccal surface, and Schiff (1867) made the important
observation that a solution of acid too weak to stimulate
the general mucous surface would nevertheless call forth
a sour sensation when it was applied to the gustatory
region. Von Vintschgau (1880) made similar observa-
tions concerning the saline taste; solutions of sodium
chloride, potassium iodide, and ammonium chloride, if
sufficiently weak, will stimulate the organs of taste, but
if strong they will stimulate not only these organs but
the nerve endings of the general buccal cavity also. In
consequence of such observations sour and saline are now
universally included with bitter and sweet as true tastes.
In addition to these four tastes there are a number
of questionable ones such as metallic and alkaline, tastes
that were originally accepted by Wundt (1887) among
others. The so-called metallic taste is excited by solu-
tions of salts of the heavy metals, silver, mercury, and
PHYSIOLOGY OF GUSTATION 135
the like (Kahlenberg, 1898). The metallic taste of a
0.0005 molar solution of silver nitrate is very pronounced
and is discernible even at the greater dilution, 0.0002.
Since the nitrate ions are incapable of exciting taste at
such slight concentrations, it follows that stimulation
must depend upon the silver ions. In a similar way mer-
cury ions in normal solutions of 0.001 to 0.0005 of mercu-
ric chloride have been shown to excite the so-called
metallic taste. This taste, however, has been declared
to be a complex of other tastes such as sour and sweet,
and Herlitzka (1808) has gone so far as to maintain that
it is not a true taste but an olfactory phenomenon.
The alkaline tastes so-called are excited by the appli-
cation to the tongue of dilute solutions of such caustic
alkalis as sodium or potassium hydrate. Kahlenberg
(1898) has shown that the stimulating material in such
mixtures is the hydroxylion which is effective in solutions
as weak as 0.0025 molar. In the alkaline taste, as in the
metallic taste, the results have been variously explained.
Oehrwall (1891) regarded the so-called alkaline taste as
a mixture of sensations due to a simultaneous combina-
tion of several tastes and touch. Hober and Kiesow
(1898) pointed out that weak alkalis produce a sweetish
taste, but von Frey (1910) showed that these reagents
act on the tongue in such a way as to produce odorous
materials that he believed to be the occasion of the so-
called alkaline taste. He, therefore, relegated these as-
sumed tastes to olfaction.
Insipidity, such as is characteristic of distilled water,
is probably real tastelessness. Oehrwall (1891) attributed
it to the absence of small amounts of carbon dioxide from
such waters and this is probably true, for tastelessness
136 SMELL, TASTE, ALLIED SENSES
disappears on the addition of some of this gas to insipid
water. Henle (1880) showed that insipidity was char-
acteristic of fluids that contained less salt than the saliva.
Insipidity is probably a deficiency phenomenon and may
be produced by the absence of several classes of sub-
stances. Nevertheless it must not be forgotten that a
condition of staleness or flatness in water, practically
indistinguishable from insipidity, can be produced by
introducing into the water very small amounts of caustic
alkali whereby hydroxyl ions are liberated (Kiesow,
4. The Sour Taste. Sour taste has long been asso-
ciated with acid substances. In fact it seems very prob-
able that the sour taste is excited only by acids, acid
salts, or materials that produce acids. All these sub-
stances on going into aqueous solution give rise to hydro-
gen ions by the dissociation of acid molecules. If the
solutions are strong they will also contain a certain
number of undissociated acid molecules. It was pointed
out by Richards (1898) that, since all such solutions have
the sour taste and since the one component that they all
have in common is the hydrogen ion, this ion must be the
occasion of their common taste. This conclusion was
independently arrived at in another way by Kahlenberg
(1898). A 0.0025 molar solution of hydrochloric acid has
a pronounced sour taste and its dissociation into hydro-
gen and chlorine ions is practically complete. A corres-
ponding solution of sodium chloride is also about
completely dissociated into sodium and chlorine ions but
is without taste. It follows, therefore, since there are
as many chlorine ions in the salt solution as in the acid
solution per unit volume and the salt solution is without
PHYSIOLOGY OF GUSTATION 137
taste, that the sour taste of the acid solution cannot be
due to its chlorine ions but must be occasioned by its only
other constituent, the hydrogen ions. Kahlenberg, there-
fore, concluded that these ions are accountable for the
This view is supported by the fact that the sourness
of all acid solutions is the same, for instance, it is impos-
sible to distinguish by taste hydrochloric acid from nitric
or sulphuric acid. So far as the sensations are concerned
all these reagents produce identical results, the one qual-
ity of sourness. There has been some tendency to sepa-
rate astringency from sourness, but it is generally
conceded that astringency is merely sourness near the
vanishing point. With hydrochloric and other mineral
acids this occurs in molar solutions at about 0.00125 to
0.001 below which the acid solutions cannot be distin-
guished from pure water.
From this standpoint sour taste might be regarded
as due directly to hydrogen ions and the intensity of this
taste to depend upon the concentration of such ions.
But the question is not so simple as this. Although solu-
tions of most mineral acids agree well among themselves
so far as sourness and hydrogen ion concentration are
concerned, organic acids are not necessarily so related.
Most organic acids are much less dissociated in aqueous
solution than are inorganic acids and contain, therefore,
in normal solution, fewer hydrogen ions per unit volume,
than inorganic acids do. Nevertheless Eichards (1898)
found that tartaric, citric, and especially acetic acids were
more sour than would have been expected from the hydro-
gen ion concentration of their solutions. According to
Richards acetic acid is about as sour as a solution of
138 SMELL, TASTE, ALLIED SENSES
hydrochloric acid one-third as concentrated. Nevertheless
the acetic acid is dissociated only about one-fourteenth as
much as the hydrochloric. Hence ion for ion the acetic
acid solution is the more sour of the two. This result
was also arrived at by Kahlenbcrg (1898) who estimated
the sourness of acetic acid at a concentration of 0.005
molar to be about four times what should be expected from
its hydrogen ion content. These differences were sub-
sequently reaffirmed by Becker and Hertzog (1907).
It is by no means easy to explain the excess of sour-
ness on the part of acetic and other like acids. Richards
has suggested, without putting great stress on the idea,
that the additional sourness of acetic acid may be due to
the undissociated molecules, which, serving as a reserve,
producing additional hydrogen ions as those present are
used up in the reaction between the acid solution and the
surface of the receptor, an opinion supported by the
recent work of Harvey (1920). Crozier (1916, 1918a,
1918b), on the other hand, has pointed out the probability
that the question is double, one part having to do with
penetration and the other with the production of the
sour taste. By taking advantage of natural indica-
tors, such as the blue pigment in the integument of
Chrompdoris, it can be easily shown that acids pene-
trate living cells. This may be assumed to be the
first step in sour gustation. But penetration observed
in this way is a much slower process than gustation,
hence the penetration concerned with taste can have
to do only with the most superficial layer of the
taste cells. It is the ease of combination with this layer
that may make the difference between acetic acid and
other acids. Different acids having penetrated the sur-
PHYSIOLOGY OF GUSTATION 139
faces of gustatory cells at different rates, their uniform
sour taste may then be ascribed to their common dissocia-
tion product, the hydrogen ion. How this is accomplished
is, according to Crozier, the second problem in gustation.
That the sour taste is in some way dependent upon hydro-
gen ions seems true beyond reasonable doubt. How these
ions become effective is still a problem.
5. The Saline Taste. The saline taste is typified by
that of common salt. Sodium chloride, however, is not
the only substance that possesses this taste, for there is
a whole range of compounds that have the same property.
The chlorides of potassium, lithium, ammonium, and mag-
nesium, the hydro chlorides of monomethylamine and of
diethylamine, the bromides and iodides of sodium and of
potassium as well as their sulphates and nitrates are
all more or less saline in taste.
Aqueous solutions of most of these salts show a high
degree of dissociation so that, beside undissociated mole-
cules, cations and anions are present in these solutions
as possible stimuli for the saline taste. Hober and Kiesow
(1898) have worked on this question and have declared
in favor of ions as the stimulating agents in contrast with
undissociated molecules. Kahlenberg (1898) arrived at
the same conclusion. He found that a solution of sodium
chloride, 0.02 molar, was scarcely to be distinguished by
taste from pure water. At 0.04 molar it was a trifle
saline. Corresponding solutions of sodium acetate were
almost tasteless and certainly not in the least saline.
Hence it is evident that the salty taste of sodium chloride
is due to chlorine ions and not to sodium ions. This con-
clusion is supported by the fact that 0.04 molar solutions
of potassium chloride and of lithium chloride are also
140 SMELL, TASTE, ALLIED SENSES
salty. Other chlorides, such as those of ammonium and
magnesium, have a saline taste.
This taste, however, is not due exclusively to chlorine
ions. Sodium bromide at 0.02 molar has a faint saline
taste and is unquestionably salty at 0.04. Hence the
bromine ion must also be a stimulus for the salty taste.
Kahlenberg (1898) reported it as not quite so effective
in this respect as the chlorine ion. Although solutions
of sodium iodide at 0.04 or even at 0.02 molar could be
distinguished from water, they did not give an unques-
tionable taste till a concentration of 0.16 was reached.
At this concentration the taste w r as markedly saline. A
corresponding solution of potassium iodide was found
also to be salty though in this instance the taste was ac-
companied by a slightly bitter flavor. Prom these con-
siderations it is evident that iodine ions are saline stimuli
though they are not so effective in this respect as chlorine
or bromine ions are. The sulphates of sodium and of
potassium as w r ell as their nitrates also have a saltiness
in their tastes and it has been shown in these instances
that the sulphate and nitrate ions are the effective agents.
Thus all saline tastes depend upon ionic stimuli, and, as
Kahlenberg (1898) and Hober and Kiesow (1898) have
maintained, these ions are always anions, a conclusion
supported by the more recent work of Herlitzka (1908).
6. The Bitter Taste. The bitter taste is character-
istic of almost all alkaloids, and of certain unrelated sub-
stances such as dextro-mannose, the glucosides, picric
acid, ether, and certain inorganic salts such as magnesium
sulphate or Epsom salt.
Magnesium salts when sufficiently concentrated have
a bitter taste and this taste is due to the magnesium ion.
This is in strong contrast with the ions of sodium and
PHYSIOLOGY OF GUSTATION 141
of lithium, which are apparently almost tasteless. Am-
monium and calcium ions are also bitter in taste. In
picric acid the sour taste of the hydrogen ion is probably
completely masked by the bitter taste of the picric anion
though the taste of this substance as well as that of ether,
dextro-mannose, the glucosides and other such substances
appears never to have been fully investigated.
But the substances that are especially characterized
by bitter tastes are the alkaloids. These include such
compounds as morphine, cocaine, pilocarpine, quinine,
nicotine, and strychnine, the bitterest of all substances.
In aqueous solution these substances are the most effec-
tive agents in exciting the bitter taste. Gley and Richet
(1885) determined that strychnine monochloride could be
tasted at 0.0006 gram per liter of water. Of such a
solution 5 cubic centimeters, which was the volume used
by these investigators in their individual tests, contains
only 0.000005 gram of the bitter material and yet this very
small amount produces a pronounced taste. Quinine
hydrochloride can be tasted in a solution as dilute as
0.00004 molar (Parker and Stabler, 1913). Thus bitter
substances far exceed hydrogen ions in their capacity to
stimulate at high dilution.
What peculiar chemical feature is characteristic of
bitter organic substances whereby they excite this taste
is at best, poorly understood. Henry (1895) pointed out
that the bitter compounds often included the group
/CH 2 OH
N0 2 -C-
and this was confirmed by Cohn (1914) whose extensive
study of the sapid organic compounds led him to the con-
clusion that there were several such groups, the presence
142 SMELL, TASTE, ALLIED SENSES
of any one of which in a given compound would give it
a bitter taste. In dyes color-radicals have long been
called chromophores ; by analogy radicals concerned with
taste have been designated saprophores. Among these
are hydroxyl and the amine group. The nitro group N0 2
is often associated, especially in aromatic compounds,
with a bitter taste. When three N0 2 groups are included
in a given compound, it always has a bitter taste ; when
two are at hand, the taste is commonly bitter but not
invariably so; when only one such group is present, the
taste is not bitter. Thus the number of N0 2 groups ap-
pears to be significant in the production of a bitter taste.
The bitter taste, then, is excited by several classes of
substances; by ions that, with the possible exception of
the anion of picric acid, are apparently always cations
Herlitzka (1908), magnesium, ammonium, and calcium;
and by organic substances, especially the alkaloids, which
may act either through their molecules or through certain
atomic groups, the so-called saprophores.
7. The Sweet Taste. The sweet taste is excited by
the diatomic and polyatomic alcohols of the aliphatic
series, by the aldehydes and ketons derived from these
alcohols, and especially by the hexoses whose polymeriza-
tion products, the disaccharides arid polysaccharides, are
in this respect particularly important. Besides these
carbohydrates other organic compounds, such as chloro-
form, dextro-asparagine, and saccharine, have sweet
tastes. Among inorganic substances neutral acetate of
lead, often called sugar of lead, and the salts of glucinum
are known to be sweet. Solutions of the alkalis, if they
are of appropriate dilution, are said likewise to excite
PHYSIOLOGY OF GUSTATION 143
What occasions the sweet taste of lead acetate seems
never to have been ascertained. On the other hand glu-
cinum chloride and glucinum sulphate, both of which
break into ions in water, have been shown by Hober and
Kiesow (1898) to owe their sweet taste to their common
constituent, the glucinum ion. Thus ions are one means
of exciting this taste.
But the sweet taste, like the bitter one, is primarily
associated with organic compounds. It centers about the
alcohols and especially the sugars in much the same way
that the bitter taste does about the alkaloids. Although
the halogenated hydrocarbon chloroform and the aromatic
compound saccharine are both sweet, the latter about 500
times as much so as cane sugar, the great majority of sweet
substances are aliphatic alcohols and their derivatives.
Ethyl alcohol is sweetish in taste as well as the trihydric
alcohol glycerol, but the type of sweet substances is cane
sugar or sucrose. This can be tasted in aqueous solution
to about 0.02 molar; in weaker concentrations it is diffi-
cult to distinguish it from pure water. Ethyl alcohol
cannot be tasted in solutions much weaker than 3 molar, a
relatively high concentration (Parker and Stabler, 1913).
What determines the sweet taste in carbohydrates is
by no means settled. It apparently turns upon very
slight differences. These are sometimes sterioisomeric
in character. Thus, as already stated, dextro-asparagine
is sweet and laevo-asparagine is tasteless. Dextro-man-
nose is sweet and its stereoisomer dextro-glucose is bitter.
Other such examples are known. In some instances
slight changes in composition are accompanied by con-
siderable changes in taste. Thus, according to Thorns
and Nettesheim (1920), dulcin loses its sweetening power
144 SMELL, TASTE, ALLIED SENSES
when acidic or basic substitutes are introduced into its
benzene nucleus. The introduction into a sweet molecule
of any considerable radical, especially an aromatic
one, is very likely to be followed by a change from
sweet to bitter.
Colin (1914) made an elaborate comparison of the
constitution of the sweet substances, as he did that of
the bitter compounds, and came to the conclusion that
these substances like the bitter ones contained particular
groups of atoms that determined their taste and that he
designated as glucogenes. Thus among alcohols one hy-
droxyl is accompanied with slight sweetness and four
or five with intense sweetness. But notwithstanding
the extent of Colin 's comparisons, Oertly and Myers
(1919) found his generalizations inadequate, and pro-
posed in place of his hypothesis one in which two groups
were assumed to be present in every sweet molecule. Fol-
lowing by analogy the terminology used for dyes, one of
these groups was called a glucophore and the other an
auxogluc. By a close comparison of the sugars, amino
acids, and halogen derivatives of the hydrocarbons, they
believed they could identify at least six glucophores and
nine auxoglucs. The glucophores are (1) CH 2 OH-CHOH-,
(2) -CO-CHOH-(H), (3) C0 2 H-CHNH 2 -, (4) -CH 2 ON0 2 ,
(5) CJ-*-, and WC^-'-C^-. The auxoglucs are (1)
H-, (2) X CH,-, (3) CH 3 CH 2 -* (4) CH,-CH 2 -CH 2 -, (5)
(CH 3 ) 2 CH-, (6) CH,OH-, (7) CH,CHOH-, (8) CH 2 0!H-
CH 2 -, and (9) radicals C^Ho^On of normal polyhe-
An illustration of the way in which Oertly and Myers'
theory may be made to apply to sweet substances is given
PHYSIOLOGY OF GUSTATION 145
in the following table in which the resolution of a number
of sweet compounds into glucophores and auxoglucs
A table of sweet organic compounds (aliphatic series) showing the
constitution of the compound and its resolution into a glucophore and an
auxogluc, from Oertly and Myers (1919).
Compound Constitution Glucophore Auxogluc
Glycol CH 2 OH-CH 2 OH CH 2 OH-CHOH H-
Glycerol CH 2 OH-CHOH-CH 2 OH CH 2 OH-CHOH CH 2 OH-
Fructose CH 2 OHCO ( CHOH ) 8 CH 2 OH .COCHOH-(H) C n H 2n +iOn-
Glycine CH 2 NH 2 -COOH -CHNH 2 -COOH H-
Ethyl nitrate C 2 H 3 ON0 2 -CH 2 ON0 2 CH 3 -
Notwithstanding the elaborate attempts of Cohn and
of Oertly and Myers to elucidate the chemoreception of
sweet substances, the subject must be admitted to be one
that is far from settled. What may be said with cer-
tainty is that the sweet taste, like the bitter taste, is ex-
cited both by ions and by organic molecules the details
of whose activity, however, are by no means fully
8. Inadequate Stimuli. Taste is somewhat remark-
able for its paucity of inadequate stimuli. Although the
tongue is very sensitive to temperature differences, these
changes do not seem to excite the gustatory receptors.
It is questionable whether mechanical stimulation, such
as tapping the tongue as practised by the older physiolo-
gists, will call forth sensations of taste. The only really
effective form of inadequate stimulus for the gustatory
organs seems to be the electric current. As early as
1752 Sulzer noted the peculiar sensations when two dif-
146 SMELL, TASTE, ALLIED SENSES
ferent metals are placed simultaneously on the tongue.
This observation was independently made by Volta in
1792 who believed these sensations to be produced by the
electrical stimulation of the organs of taste, for he ob-
tained the same effects by passing an electric current
through the tongue. Five years later, however, Humboldt
pointed out that the real stimulating agent in the
so-called electric taste might be the substances produced
by electrolysis at the region where the current passes
from the electrode into the tongue rather than the electric
current itself. Thus was established the two opposing
views concerning electrical taste.
If an electric current is passed through the human
body in such a way that the anode is applied to the tongue
and the cathode to some other part, a sour taste develops
around the anode. If the electrodes are reversed in posi-
tion, an alkaline taste appears at the cathode. This con-
forms with what takes place when an electric current is
passed through an alkaline solution, such as the saliva;
hydrogen ions appear at the anode and hydroxyl ions
at the cathode. Why then are not these two substances,
the hydrogen and the hydroxyl, the stimuli for the char-
But Rosenthal (1860) and, before him, Volta, found
that if the anode is a weak alkaline solution into which
the tip of the tongue is dipped, a sour taste nevertheless
arises, though the hydrogen ions under such a combination
might be expected to be neutralized immediately by the
hydroxyl present. Rosenthal also showed that if an elec-
tric current is passed through the bodies of two persons
and is completed by bringing the tip of the tongue of one
of these individuals into contact with that of the other,
PHYSIOLOGY OF GUSTATION 147
the two persons experience different sensations, one sour
and the other alkaline. These and other like experi-
ments led Eosenthal to conclude that the electric current
itself was the stimulating agent and not the materials
produced by electrolysis.
But it must not be forgotten that the electrical stimu-
lation of organs of taste is productive of a variety of
sensations. Thus in 1798 Ritter showed that after a cur-
rent had been passing for some time through the tongue
the sour taste of the anode changed first to bitter and then
to alkaline while the cathodic alkaline taste changed to
sour. Hofmann and Bunzel (1897) demonstrated that
during the passage of a current there is at the cathode
a burning bitter sensation which changes to a sour metal-
lic taste on breaking the current. The initial taste they
believed to be due to the products of electrolysis. Von
Zeynek (1898) also accepted this explanation for the elec-
tric taste, Gertz (1919), however, pointed out that the
alternating current is really more effective in exciting
taste than the direct current and that hence the electric
taste may be aroused by other means than the products
of electrolysis. It is not at all impossible that the gusta-
tory organs may be excited in both ways : by the materials
of electrolytic decomposition and directly by the electric
current. But how an electric current can stimulate gus-
tation without in some way bringing about a chemical
change, at least within the gustatory cell, is difficult
The extreme sensitiveness of the organs of taste to
electrical stimulation is not only characteristic of man,
but is probably found throughout the vertebrates.
Among fishes the catfish or horned pout, Amiurus, is ap-
148 SMELL, TASTE, ALLIED SENSES
parently easily stimulated in this way. The head and
especially the eight barbels about the mouth of Amiurus
are richly supplied with taste-buds. These organs, like
those on the human tongue, are apparently extremely
sensitive to metals probably because of the slight electric
currents produced by these bodies, for, the fishes respond
with great readiness to a weak constant current from a
dry cell. If such a current is led into an aquarium
through a water-filled glass tube and out again by a sim-
ilar tube, the water acting as a conductor, catfishes can
be readily stimulated by bringing such tubes close to
them. If the current is sufficiently reduced (a little less
than a microampere) the fishes will approach the open
ends of the tubes and nibble at the current as though it
were a bait, thus giving evidence that the organs stimu-
lated are the gustatory receptors (Parker and Van
Heusen, 1917). Hence the electric stimulus seems in every
way to duplicate the stimulus normal for the organ of
taste, a solution of a sapid substance.
9. Distribution of Tastes on the Tongue. The four
well-recognized tastes, as the preceding sections show,
are normally excited by very different stimuli. The sour
taste is dependent upon the cation, hydrogen. The saline
taste is called forth by a number of anions : chlorine,
bromine, iodine, and the sulphate and nitrate ions. The
bitter taste has as stimuli the alkaloids, such cations as
magnesium, ammonium, and calcium, and possibly the
anion of picric acid. The sweet taste depends upon such
organic compounds as the sugars and alcohols, and on
saccharine, on lead acetate, and on hydroxyl and gluci-
num ions. The four tastes, therefore, are excited by
entirely independent groups of stimuli and it is a matter
PHYSIOLOGY OF GUSTATION
of importance to ascertain in what other respects they
are independent. This question can be well approached
from the standpoint of their distribution on the tongue.
As already mentioned, the tongue of the normal adult
human being is only in part gustatory, its lower surface
and the central portion of its upper surface being
A B C D
FIG. 36. Diagrams of the right half of the human tongue illustrating the distribution of
the four tastes; the dots represent the area and concentration of a given taste: A, the sour
taste, concentrated on the edge; B, the saline taste, concentrated at the tip and on the edge;
C, the bitter taste, concentrated at the base; D, the sweet taste, concentrated at the tip.
Modified from Hanig, 1901.
devoid of taste. This sense is resident only on the
tip, the edges and the dorsal part of the root of the
tongue. The distribution of the several tastes over the
gustatory portion of the tongue has been a matter of
investigation for physiologists during more than a cen-
tury, and the results, particularly among the recent
workers, have been remarkably consistent and harmoni-
ous. Shore (1892), Kiesow (1894-1896), and Hanig
(1901) have been the most important recent contributors
150 SMELL, TASTE, ALLIED SENSES
to this subject. Their work shows that the four tastes
have decidedly individual distributions on the tongue.
The sour taste is best developed on the lateral edges of
the tongue and diminishes from these regions toward the
tip, the base, and the central anaesthetic area (Fig. 36, A).
The saline taste is most pronounced at the tip and on
the lateral margins of the tongue and diminishes at the
base; toward the central area it ends rather abruptly
(Fig. 36, B). The bitter taste is most characteristic of the
base of the tongue especially in the region of the vallate pa-
pillae whence it diminishes rapidly toward the central area
and over the lateral edges to the tip (Fig. 36,C). The sweet
taste is at its maximum at the tip of the tongue and di-
minishes thence along the lateral margins to the base (Fig.
36, D). Thus sour is represented by two marginal re-
gions, saline by a horse-shoe shaped area at the tip, bit-
ter by a single center at the base, and sweet by one at
the tip. It is difficult to explain these differences in the
distribution of the tastes except on the assumption of
an independent sensory mechanism for each taste.
This interpretation of taste is strengthened by what
has been learned from the local stimulation of the tongue.
Oehrwall (1891) mapped out a group of fungiform papillae
near the tip of the tongue in such a way that the
individual papillae could be reidentified and studied.
Each papilla was stimulated by applying to it the point
of a very fine brush loaded with a strong solution of a
given substance. The substances used were tartaric
acid 2 per cent, common salt 20 per cent, quinine hydro-
chloride 2 per cent, and sugar 40 per cent. The salt was
finally abandoned because of the indistinctness of the
sensation. In all 125 easily identifiable papillae were
PHYSIOLOGY OF GUSTATION 151
tested. All of these were found to be sensitive to touch,
warmth, and cold, but only 98 were stimulated by the
solutions used. The results of these tests so far as they
relate to taste are given in the following tabulation.
Test Substances Acid Quinine Sugar
Number of papillse sensitive 91 71 79
Number exclusively sensitive! 12 3
The fact that 12 papillae were stimulated by tartaric
acid but not by quinine or sugar and that 3 were stimu-
lated by sugar but not by tartaric acid or quinine is strong
evidence in favor of the independence of at least the sour
and sweet tastes. Oehrwall also discovered by his
method of local stimulation that the surface of the tongue
between the papillae was insensitive to taste.
This result confirmed the earlier work of Goldscheider
and Schmidt (1890) who had also shown that when pa-
pillae were tested with a mixed solution of sugar and
quinine sometimes a sweet taste was evoked and at other
times a bitter one.
This whole subject was thoroughly re-investigated by
Kiesow (1898), who used as stimuli solutions of hydro-
chloric acid, of sodium chloride, of quinine sulphate and
of sugar. Of the 39 papillae tested 4 were found to be
insensitive. The conditions presented by the remaining
35 are summarized in the following tabulation.
Test Substances Acid Salt Quinine Sugar
Number of papillae sensitive 18 18 13 26
Number exclusively sensitive 3 3 7
Failed of stimulation 17 17 22 9
These results confirm and extend the original findings
of Oehrwall in that they show the independence of the
sour, saline, and sweet tastes. The fact that the region
152 SMELL, TASTE, ALLIED SENSES
tested was near the tip of the tongue is probably the
occasion of the absence in the records of any papillae
stimulated exclusively by quinine, for this region is one
in which the bitter taste is least developed. Kiesow also
observed that the papillae presented a great variety of
combinations in taste ; some were open to stimulation by
two of the four reagents used, others by three and still
others by all four. It is known that each gustatory
papilla carries a number of taste-buds but whether in
those papillae that are open to stimulation by two or more
sapid solutions there is a corresponding number of kinds
of buds, one for acid, another for salt reception and so
forth, cannot be stated, for it is possible that this dif-
ferentiation may reach to the gustatory cells of each
bud. What can be affirmed, however, is that in those
papillae that respond exclusively to one taste all taste-
buds with their contained cells must be so constituted as
to be open to stimulation by one class of sapid substances
and to be closed to all other classes. Thus in a papilla
that is stimulated exclusively by acid the protoplasm of
the receptive cells in all its taste-buds must be organized
to receive acid stimuli and not to react to those for the
saline, bitter and sweet tastes. This conclusion amounts
to a very complete confirmation of Miiller's theory of the
specific energy of sensory nerves as it is interpreted in
modern physiology and to the component theory as ap-
plied to taste.
10. Action of Drugs on Taste. Certain drugs have
the remarkable property of temporarily diminishing or
even obliterating taste. Edgeworth discovered that af-
ter a person had masticated the leaves of the Indian
asclepiad Gymnema sylvestre, he was unable to taste
PHYSIOLOGY OF GUSTATION 153
sugar. Hooper (1887) extracted from the leaves of this
plant a compound that he named gymnemic acid and that
he showed to be the substance that affected taste. Ac-
cording to him gymnemic acid tends to obliterate the
sweet and bitter tastes but has no effect on the saline and
sour tastes. Shore (1892) studied the influence of gym-
nema decoctions on the tongue and found that they oblit-
erated the sweet taste of glycerine very easily and the
bitter taste of quinine almost as readily. They had very
little effect on the taste of sulphuric acid or of common salt.
These results were confirmed in the main by Kiesow
(1894). Thus gymnemic acid divides the tastes into at
least two distinct classes, one including sweet and bitter,
and the other sour and saline.
Stovaine is also known to abolish sweet and bitter
without obliterating saline and sour (Ponzo, 1909) and
eucaine-B especially reduces bitter (Fontana, 1902).
Saline and sweet tastes and in less degree bitter are
reduced by a 0.02 normal solution of chromium nitrate
The effect of cocaine on taste is very profound. Von
Anrep (1880) and Knapp (1884) observed that this nar-
cotic was capable of abolishing completely all taste.
Aducco and Mosso (1886) showed, however, that it acted
more energetically on the bitter taste than on the others.
Shore (1892) found that on treatment with cocaine the
buccal sensations were extinguished in a definite order
as follows : pain, bitter, sweet, saline, sour, and touch, a
sequence confirmed by Kiesow (1894). Thus cocaine is
more selective in its effect on taste than gymnemic acid
and leads to a separation of all four tastes.
11. Substances with two Tastes. A number of sub-
154 SMELL, TASTE, ALLIED SENSES
stances are known that possess different tastes depend-
ing upon the part of the tongue to which they are applied.
Many salts have this peculiarity Herlitzka (1908).
Potassium nitrate and magnesium sulphate are both said
to be saline in taste when applied at the tip of the tongue
and bitter at its base. This action, however, is proba-
bly due not to the molecules of the salts but to their ions.
At the tip of the tongue the anions stimulate the organs
of the saline taste, which in this location are in the ascen-
dency, and at the base of the tongue the cations stimulate
the organs of the bitter taste which is here better devel-
oped. There is thus a kind of competition between the
two sets of ions, as Herlitzka has expressed it, and in one
locality the anions win out, in the other the cations.
Such an explanation, however, does not apply to sub-
stances like parabrombenzoic sulphinide. This material,
according to Howell and Kastle (1887) has a distinctly
sweet taste when applied to the tip of the tongue and an
intensely bitter one at the back. Dulcamarin, the gluco-
side from bittersweet, is another case of the same kind; a
list of these is given by Sternberg (1898). In these in-
stances ions are probably not involved, but each substance
is a stimulus for both the organs of the sweet taste and
of the bitter taste. It seems impossible to explain double
tastes such as those just mentioned except on the assump-
tion of independent receptor systems for the tastes con-
cerned. Thus far no substance is known that excites
three categories of tastes though I know of no reason why
such a substance might not exist.
12. Latency of Taste Sensations. Von "Wittich
(1868) appears to have been the first to attempt to meas-
ure the interval of time between the application of a
PHYSIOLOGY OF GUSTATION 155
stimulus to a gustatory portion of the tongue and the
response of the subject. He used an electric current as a
stimulus and found the average time to be 0.167 seconds.
Von Vintschgau and Honigschniied (1875-1877), who used
solutions of various substances as stimuli, found that the
times were different for the different tastes, being short-
est for saline, longer for sweet, still longer for sour and
longest for bitter. They also discovered that the times
were different for the tip of the tongue and its base.
Their results were confirmed in general by the later in-
vestigations of Beaunis (1884), of Henry (1895) and of
Kiesow (1903) who recorded the following periods for
the tip of the tongue :
Sodium chloride 0.308 second
Sugar 0,446 second
Hydrochloric acid 0.536 second
Quinine 1.082 second
These records agree with Schirmer's observation
(1859) that when a solution containing all four sapid sub-
stances is placed on the tongue, the subject experiences
the sensations in the order saline, sweet, sour, and bitter.
They also confirm the opinion that the four tastes are
One aspect of the problem of gustatory latency turns
on temperature. If the stimulation of a taste receptor
is a chemical operation, this process should exhibit a con-
siderable temperature co-efficient that might make itself
felt in a change in the latent period. But so far as I am
aware no studies with this point in view have been car-
13. Taste Alterations; After-tastes. A number of
156 SMELL, TASTE, ALLIED SENSES
substances are known whose solutions so affect the tongue
that its powers of taste become temporarily changed.
Thus these substances give rise to what have been called
after-tastes. In almost every instance the taste that
suffers change is the sweet taste and this is increased in
efficiency. Thus Aducco and Mosso (1886) found that
after the tongue had been held in dilute sulphuric acid
for five to ten minutes, distilled water was then capable
of exciting a very sweet taste. A solution of quinine was
also sweet to the taste at the tip of the tongue, but it
remained normally bitter at the base. This change was
not brought about by other acids such as acetic, citric, and
formic. Frentzel (1896) also noticed that after washing
out the mouth with a weak solution of copper sulphate,
smoking a cigar was accompanied by a sweet taste. Ac-
cording to Zuntz (1892) a solution of sodium chloride of
one per cent strength will increase the sweetness of sugar,
an observation confirmed by Heymans (1899). A mouth
wash of potassium chlorate is well known, to leave the
tongue so that distilled water tastes sweet (Nagel, 1896).
In all these instances it is probable that the constitution
of the receptor for the sweet taste is so changed by the
first solution applied to it that it becomes hypersensitive
to its normal stimuli such as sugar or even open to novel
stimuli such as distilled water.
Complete loss of taste or ageusia is known to accom-
pany hysterical and other abnormal nervous states. It
may be temporary or, in the case of certain lesions, per-
manent in character.
14. Gustatory Contrasts. Although some acids in-
crease the sensitiveness of the sweet taste and thus give
ground for a gustatory contrast, it is questionable whether
PHYSIOLOGY OF GUSTATION 157
such contrasts exist as extensively as was believed by the
older workers. It is a common opinion that after a sweet
drink a sour taste is more intense, but Oehrwall (1891)
was unable to confirm this experimentally nor could he
show that bitter increased the sensibility to sweet.
Haycraft (1900) noted that when one border of the tongue
is rubbed with salt, the other border becomes hypersensi-
tive to sugar, but such a contrast is clearly not peripheral
but central in origin, and possibly other contrasts may
be thus explained.
15. Taste Compensations and Mixtures. Mixtures
of sapid solutions do not as a rule give rise to tastes other
than those of their components. Lemonade has both the
sweet taste of the sugar and the sour taste of the citric
acid it contains. Sugar adds a pleasant element to cof-
fee, but does not destroy its bitter taste. In ordinary
food the flavor is the mixture of true tastes and odors
accompanied by the multitude of other buccal sensitivities
due to the variety of substances in the mouth and accep-
ted in a rather unanalyzed form by the central apparatus.
Yet in all this complexity the elements remain essentially
distinct. Competition rather than compensation seems
to be the rule. Kiesow (1894-1896) , however, has claimed
that a very weak solution of sugar and salt gives a taste
that is neither sweet nor saline but distinctly flat, and
Kremer (1918) has recently shown that a solution of
sodium chloride too weak to stimulate the saline taste
will, nevertheless, considerably increase the sweetness of
a cane-sugar solution. Quinine hydrochloride on the
other hand will, according to Kremer, reduce sweetness.
These instances may be evidence of gustatory compensa-
tion, but it seems much more probable, as was indicated
158 SMELL, TASTE, ALLIED SENSES
in a preceding section, that they result from a sensitizing
or a desensitizing of the sweet receptors by the sodium
chloride or the quinine, for it is extremely doubtful, as
Oehrwall (1891) has stated, whether true gustatory com-
pensation ever occurs. Ionic antagonism such as Crozier
(1915) has discovered in the reaction of the frog's foot to
salt solution has thus far not been identified in taste.
16. The Gustatory Senses. When a general survey
of the so-called sense of taste is made, the most striking
feature that appears is the remarkable independence of
the four categories, sour, saline, bitter, and sweet. These
are excited by groups of different stimuli, they give re-
markable evidence of having separate receptors, they are
differently acted upon by various drugs, and they show
numerous other peculiarities that are interpretable only
from the standpoint of organic separateness. So im-
pressed was Oehrwall (1891, 1901) with these peculiari-
ties that he declared them to be in all essentials four
separate senses, a declaration entirely in accord with the
component theory as applied to taste. Although this
view has a certain radical element in it and has not been
favorably received by such workers as Kiesow, Nagel,
Luciani, and Henning, who have declared for the unitary
nature of taste, it is difficult to say why it should not pre-
vail. It has been urged that gustatory compensation is
inconsistent with Oehrwall 's hypothesis and possibly this
may be true. But gustatory compensation is so uncer-
tain a phenomenon that when compared with the sub-
stantial body of evidence in favor of the hypothesis, this
objection lacks force. Henning (1916) has declared that
the tastes of different substances, members of one cate-
gory, are not necessarily alike; thus the saline tastes of
PHYSIOLOGY OF GUSTATION 159
sodium chloride, sodium iodide, and sodium bromide,
though much the same are still characteristically different.
And he has further maintained that the mixed tastes so-
called cannot be imitated by real mixtures; thus the
bitter-saline taste of magnesium chloride cannot be repro-
duced, he believes, by a mixture of sodium chloride and
bitter aloes. But all such statements imply that the
conception of the receptive independence of tastes neces-
sarily involves the further view that a gustatory stimulus
is limited to one category of receptors. That some sub-
stances, such as parabrombenzoic sulphinide, stimulate
two categories of receptors has already been made clear
and though most stimulating materials influence in a
vigorous way only one set of end-organs, it is more than
probable that they all affect at least to a slight degree
other such sets. The taste of any substance then is not
necessarily one of the four tastes and this alone, but one
of these qualified by traces of other tastes excited slightly
and simultaneously by the same stimulating agent.
Hence any substance such as sodium chloride, or sodium
bromide, may perfectly well have a somewhat individual
taste without doing violence to the hypothesis that there
are four separate tastes, and the success with which
mixed tastes so-called may be imitated is rather a matter
of skill than despair.
It is time that gustation is a strikingly unified oper-
ation, but when this unity is looked into, it is seen to
depend upon simultaneousness of action rather than on
interdependence of activities. Smell is related to taste
in much the same way that one taste is related to another.
On the whole it would seem more consistent with fact to
speak of the sour sense, the saline, the sweet, and the
160 SMELL, TASTE, ALLIED SENSES
bitter sense than of the sense of taste. Just as the sense
of feeling in the skin has been shown to consist of at
least four senses, touch, pain, heat, and cold, so taste must
be regarded as composed of at least four senses. That
these act together and in everyday experience produce a
unified effect upon us is no more reason for classing them
as one sense than in the case of the integumentary senses.
The sense of taste must, therefore, be regarded as a ge-
neric term under which at least four true senses are gath-
ered: sour, saline, bitter, and sweet (Oehrwall, 1891, 1901).
Although the sense of taste thus loses a certain amount
of its reality, the senses classed under it probably possess
a kind of genetic unity that is not without significance.
It is very probable that these four senses represent four
lines of differentiation that have evolved from a single
ancestral sense. The remarkable uniformity of their
structure is suggestive of this view. If the four senses
under discussion have had some such origin as that just
indicated, the term sense of taste might well apply to
that primitive state, perhaps represented in some of the
lower vertebrates today, from which the four gustatory
senses of man have been derived.
17. Comparative. The comparative physiology of
taste in vertebrates is almost an untouched field. The
distribution of taste-buds in the vertebrate classes indi-
cates the presence of this sense in the mouth regions in
forms as low as the amphibians. In fishes Herrick ( 1903)
lists over thirty -five species in which taste-buds are known
to occur on the outer surface of the animal as well as
in the mouth. The catfish Amiurus is remarkable in this
respect in that its whole outer surface is provided with
these organs which are most abundantly present on the
PHYSIOLOGY OF GUSTATION 161
barbels. When a piece of meat is brought into contact
with the barbel of one of these fishes, the animal will
immediately seize and swallow the morsel. The same is
true when the meat is brought in contact with the side of
the fish. This quick seizure and swallowing of the food
has been called by Herrick the gustatory response. If
a barbel or the flank of Amiurus is touched with a pledget
of cotton instead of the meat, the fish will turn toward
the object, but, as a rule, will not snap at it. This Herrick
has designated the tactile response. If, now, the
cotton is soaked with meat juice and brought to the side
of the fish, the quick gustatory response follows. The
same form of response is made to meat juice discharged
from a pipette on the side of the fish. From this and
other tests Herrick concluded that the gustatory response
in Amiurus could be called forth by purely gustatory
stimuli unaccompanied by touch and that for this fish
taste is accompanied by a local sign as touch is. That
these responses are really gustatory is shown by the fact
that when the branch of the seventh nerve that innervates
the taste-buds on the flank of Amiurus is cut, the re-
sponses no longer occur (Parker, 1912).
Conditions similar to those in Amiurus were recorded
by Herrick in a number of gadoid fishes and it is thus
clear that taste is a general integumentary function in
many of these animals. To what extent the taste-buds of
the fish skin are differentiated for the several senses of
sour, saline, bitter, and sweet cannot be stated. It is
remarkable, however, that in almost all the fishes tested
no response to sugar has been found not only on the sur-
face of the body but also in the mouth (Parker, 1912).
162 SMELL, TASTE, ALLIED SENSES
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INTERKELATION OF THE CHEMICAL SENSES.
Contents. 1. Common Features in the Stimulation of
Chemoreceptors. 2. Differences among Chemoreceptors.
3. Groups of Chemoreceptors. 4. Classification of Recep-
tors in General. 5. Genetic Relations of Chemoreceptors.
1. COMMON Features in the Stimulation of Chemo-
receptors. The sense organs that have been discussed in
this volume, the olfactory organs, the vomero-nasal or-
gans, the common chemical receptors, and the organs of
taste, form a more or less natural group of organs under
the general title of chemical receptors or Chemoreceptors.
This designation is justified by the fact that in stimula-
tion these several types of receptors present certain
important features in common. In all instances they
are activated by solutions. This is most obvious in the
sense of taste whose stimuli from fishes to mammals con-
sist of materials in solution either in the water that enters
the mouth or in the saliva that is mingled with the crushed
food. An aqueous solution is also the stimulus for the
common chemical receptors. The nasal cavities of fishes
are likewise bathed by a continuous stream of water that
carries the stimulating substances to the olfactory sur-
faces. And in the air-inhabiting vertebrates, as already
pointed out, the olfactory terminals are probably not
exposed in any direct way to the air that carries the stim-
ulating material but are immersed in mucous through
168 SMELL, TASTE, ALLIED SENSES
which this material must make its way before it becomes
effective. In olfaction, moreover, it is probably not
simply a question of aqueous solution but, as already ex-
plained, one of solution in oil as well, for the olfactory
stimulus seems to be a material that must reach its recep-
tors through an aqueous medium that covers them and
then enter them through their lipoid components. What
has been said of the stimulation of the olfactory organ
is probably true of the vomero-nasal organ also. Thus
in one way or another all appropriate stimuli of the so-
called chemoreceptors are materials in solution.
But not all soluble materials stimulate the chemorecep-
tors. Thus such elementary gases as hydrogen, oxygen,
and nitrogen are odorless and tasteless, and a number of
organic substances have no stimulating capacity for
these organs. Those substances that do stimulate, as
was pointed out especially in the case of taste, fall into
groups whose characteristics are chemical and not phys-
ical and, though such an analysis cannot at present be
made with certainty for smell, it has already been pointed
out that the variety of smells can be explained only on a
chemical basis. Thus chemoreceptors are stimulated not
simply by material in solution, but by the chemical activ-
ity of dissolved material. On this assumption it is nat-
ural to expect that there would be a certain number of
substances, chemically inert toward the given receptors,
that would, therefore, be incapable of acting as stimuli
for them. Such substances as the gases already men-
tioned probably represent this group.
The stimulus for the chemoreceptor, however, is not
only a solution of a chemically active material, but it is
such a solution applied directly to the terminal organ.
INTERRELATION OF THE CHEMICAL SENSES 169
This peculiarity of the chemoreceptors is in strong con-
trast with that which occurs in the so-called mechanicore-
ceptors, the organs of touch, pressure, and hearing. In
these organs the appropriate stimulus is a deforming
pressure which may be exerted by an impinging or vi-
brating material that does not necessarily touch the
terminal organ itself, but may act through a considerable
amount of intervening tissue. Hence the mechanicore-
ceptors are not necessarily exposed directly to what is
ordinarily called the stimulus as chemoreceptors are, but
they may be excited more or less indirectly. Our organs
of touch and of hearing, therefore, may be lodged in the
deeper part of the skin or the head without interfering
in any serious way with their efficiency. All chemorecep-
tors on the other hand are necessarily either upon the
exposed surfaces of the body or are provided with pores
that lead from these surfaces directly to the receptors
themselves. This condition is in a way merely a corollary
of what has already been stated about chemical stimula-
tion, for if the organs of smell, taste and the like are
acted on chemically by their appropriate stimuli, these
stimuli must of necessity come into direct contact with
the given terminals.
2. Differences among Chemoreceptors. The chemore-
ceptors agree then in the general character of their stim-
uli. Such stimuli are certain chemically active materials
in solution applied directly to the receptors themselves.
The variety that these organs exhibit ought, therefore,
to turn more or less on the extent of their differentiation
in relation to the chemical diversity of the environment.
The degree of this organic differentiation, however, has
been very inadequately worked out. Almost nothing is
170 SMELL, TASTE, ALLIED SENSES
known of the stimuli for the vomero-nasal organ, and
very little has been done on those for the common chemi-
cal sense. The senses of smell and of taste are naturally
much better known. When their stimuli are compared
they are found in general to belong to different categories
of material; what is smelled is generally not tasted and
what is tasted is not smelled.
These two categories of substances afford an impor-
tant basis for comparing taste and smell. This can be done
from the standpoint of the minimum concentrations of
materials that serve as stimuli for the two sets of recep-
tors. Bitter substances are apparently the most effec-
tive stimuli for the sense of taste. Quinine hydrochloridc
can be tasted in a solution as weak as 0.00004 molar,
but this threshold is exceeded by that of what is probably
the most bitter of all substances strychnine. According
to Gley and Richet (1885) the weakest solution in which
the bitter taste of strychnine hydrochloride can be distin-
guished contains only 0.0004 gram of this substance in
one liter of water. This is approximately equivalent to
one and a half million ths of a molar solution (1.48xlO~ 6
molar), and much exceeds in this respect the efficiency
of quinine. One of the strongest odors known is that of
mercaptan of which according to Fischer and Penzoldt
(1886), 0.01 milligram evaporated in 230 cubic meters of
air gives a perceptible smell. Assuming the substance
used by these investigators to have been methyl mercap-
tan, such a dilution would bo represented by about a
million-millionths molar solution (9xlO Ki ) or approxi-
mately one and a half million times more dilute than the
weakest solution of strychnine that can be tasted. Thus
the olfactory receptor is open to stimulation by a very
INTERRELATION OF THE CHEMICAL SENSES 171
much weaker concentration than the gustatory one is.
It might be maintained, however, that the line of ar-
gument used in the last paragraph is invalid because it
is based upon measurements of one substance for taste
and another for smell, and that, therefore, the two sets
of figures are not fairly comparable. But the conclusion
just reached is also supported by determinations made
with a single substance. Ethyl alcohol is soluble in both
water and oil and is one of the relatively few substances
that has at once both taste and smell. As a matter of
fact it is also a stimulus for the common chemical sense.
Hence it may be conveniently employed for comparing all
three classes of receptors. When such a test is made,
it is found that the weakest concentration of alcohol vapor
that can be smelled is about 0.000125 molar and that the
weakest aqueous solution of this substance that can be
tasted is 3 molar. To stimulate the common chemical
sense with ethyl alcohol requires an aqueous solution of
strength 5 to 10 molar. Hence so far as ethyl alcohol
is concerned smell may be said to be about 24,000 times
more delicate than taste and about 60,000 times more
delicate than the common chemical sense. From the
standpoint of a single substance then, smell must be ad-
mitted to be vastly more efficient than either taste or
the common chemical sense both of which lie in this re-
spect close together (Parker and Stabler, 1913). Unfor-
tunately the stimulation of the vomero-nasal organ has
not yet been studied so that its capability from this
standpoint is not known, but, judged from its structure, it
probably has a receptive efficiency not far from that of
the olfactory organ. In that case the chemoreceptors of
vertebrates would fall into two groups, the olfactory and
SMELL, TASTE, ALLIED SENSES
voniero-nasal organs with high efficiency and the common
chemical receptors and organs of taste with relatively
low efficiency. These two sets of organs might in this
respect be compared with scales, the organs of taste and
of the common chemical sense resembling ordinary scales
on which only gross amounts are weighed and the organs
of smell and the vomero-nasal organs resembling chemi-
cal balances on \vhich small weights may be determined.
As olfaction deals effectively with very minute
amounts of substance and gustation only with much
greater amounts, it follows that materials that have be-
come highly attenuated by being broadly spread from
their sources either in water or in air may nevertheless
still be concentrated enough to stimulate the organs of
smell though they can have no possible effect upon those
of taste. Such faint odors are the means whereby ani-
mals scent their food, find their mates, or avoid their
enemies. Hence the olfactory organ has been appropri-
ately classed as a distance receptor or exteroceptor, to
use a convenient term from Sherrington (1906), in that
the impulses to which it gives rise commonly direct the
animal toward distant points or away from them.
Taste and, in the higher vertebrates at least, the com-
mon chemical sense are stimulated only by relatively con-
centrated solutions such as occur in connection with the
food. Hence the responses that these organs call forth
are concerned with the swallowing of food, with the re-
jection of material taken into the mouth, with mastication
and saliva and the like. These receptors are, therefore,
rightly classed as interoceptors though it must be re-
membered, as Herrick (1918) has pointed out, that in
some fishes, such as the catfishes, taste-buds serve in the
INTERRELATION OF THE CHEMICAL SENSES 173
discovery of food as well as in its appropriation, and
partake, therefore, more or less of the nature of extero-
ceptors. Although olfaction has a function independent
and separate from that of gustation in scenting mates
or enemies and gustation has a function independent of
olfaction initiates the feeding reflexes both muscular and
noxious material, both senses are intimately associated
in feeding. Food is found and the digestive secretions
are started through smell; it is swallowed and these se-
cretions are intensified ordinarily through taste. Thus
olfaction initiates the feeding reflexes both muscular and
secretory and gustation reinforces and completes them.
It is remarkable that in some fishes like the catfishes
(Amiurus) and especially the dogfishes (Mustelus;
Parker, 1914) feeding scarcely ever occurs, even when the
fishes are starving and food is present, unless the process
is initiated through olfactory reflexes. These seem to
be essential for that chain of events that result in the
final swallowing of the food, a condition that shows how
intimately smell and taste are interwoven in the verte-
Smell and taste, though thus most closely involved in
the feeding reflexes, are nevertheless perfectly distinct.
As long ago as 1821 Cloquet (Larguier des Bancels, 1912)
showed that on closing the nose by pinching the nostrils
smell can be eliminated and only taste remains. Under
such circumstances it is surprising to those who have
not previously tried the experiment to discover how small
a proportion of our food sensations are due to taste and
how large a one to smell. A cold in the head commonly
eliminates smell and leaves taste. It reduces a person
to a state in which food is often described as without
174 SMELL, TASTE, ALLIED SENSES
flavor, for only sour, saline, sweet, and bitter tastes can
be sensed and onion produces the same sweetish taste
that apple does. The separateness of smell and taste
depends doubtless upon the conditions already described.
Smell is excited in general by one set of substances ; taste
by another. Smell calls for only very weak solutions;
taste requires relatively strong ones. It may also be that
these two senses differ in the nature of the solutions that
activate them; taste is attuned to substances that form
aqueous solutions, smell to those that dissolve in oil.
Cell surfaces are commonly believed to be diphasic in
that they are composed of a mixture of two materials one
oily and the other aqueous. The gustatory hairs may be
so constituted that the aqueous constituent is the avenue
of entrance for the stimulating substance and the olfac-
tory hairs so that the oily one is the inlet. If such is the
case, this feature may also be an important difference
between smell and taste.
3. Groups of Chemical Receptors. Taste and smell
are two of the five senses ordinarily attributed to man.
But in the detailed study of the human senses not one
has escaped a kind of functional subdivision whereby it
has been shown to be more than a single sense. Thus the
internal ear originally regarded by physiologists as
purely an organ of hearing, was shown by Flourens in
1828 to be concerned in a most important way with bodily
equilibrium. From this standpoint the ear takes on the
character of a double sense organ. This duplicity is
especially well marked in certain fishes in which the
membranous labyrinth is completely divided in two cor-
responding to the functional differentiation already in-
dicated; one of these parts consists of the utriculus with
INTERRELATION OF THE CHEMICAL SENSES 175
its three semicircular canals and has to do with equi-
librium and the other of the sacculus and its appended
lagena and is concerned with hearing. Even so unified
an organ as the human eye is made up of an intermingling
of two receptive fields, for, as originally suggested by
Schultze (1866) and as elaborated by von Kries (1904),
the retinal rods are concerned with colorless vision in
dim light and the cones with color vision in bright light.
Thus the eye is differentiated for two kinds of sight, one
by night and the other by day. The integumentary sense
originally supposed to be unitary, was shown by Blix in
1884 to consist of at least three senses, cold, warm, and
pressure. To these were added in 1896 by von Frey a
fourth, pain. Thus it is clear that the conception of five
senses for man is wholly inadequate and though numbers
are perhaps not the best way of indicating the sensory
equipment of human beings or in fact of any other ani-
mal, it is not without interest to record the opinion of
Herrick (1918) that the classes of human receptors are
now known to be more than twenty.
The chemoreceptors, represented in the older accounts
by the organs of taste and smell, have no more escaped
this process of increase than have the other sense organs.
The vomero-nasal organ appears to be a kind of accessory
receptor for smell and the common chemical sense is ap-
parently a primitive form of gustatory organ. But in
addition to these subsidiary receptors, the true olfactory
surfaces as well as the gustatory areas are not homo-
geneous, but are marked by local receptive differentiation.
This is especially well illustrated by the so-called sense
of taste. This, as has already been shown in the preceding
chapter, is in reality not a single sense, but, in accordance
176 SMELL, TASTE, ALLIED SENSES
with Oehrwall's opinion (1901), must be regarded as
generic and to consist of at least three and probably four
senses, namely the sense of sour, of saline, of bitter, and
of sweet. These senses are really distinct and separate.
They have independent receptors and give rise to sensa-
tions that do not intergrade. Their association under
one head as members of the sense of taste is in a way a
misconception due doubtless to the fact that in ordinary
activity all four senses are commonly in operation at
once, and hence acquire a certain degree of functional as-
sociation. Taste then is not the name for a single sense
but for a group of senses and it is likely that smell is of
the same nature, but until olfaction is better understood,
it is impossible to indicate the elements of which it is com-
posed. Thus the chemical senses, like the others already
briefly enumerated, show the same tendency to increase
in number as they become better known.
4. Classification of Receptors in General. A detailed
investigation of the chemoreceptors leads to an increas-
ing multiplicity of elements as in the other receptor sys-
tems, and raises the question of what constitutes a unitary
sense and how such units are related. When one or
more similarly organized receptors are excited to activity
by a single category of stimuli and give rise to the same
kind of sensation we think of the aggregate as a sense.
Thus when a deforming pressure impinges upon any part
of the skin, touch receptors are stimulated and we re-
ceive a uniform impression characteristic of the sense of
touch. Or when one of a variety of sounds falls upon
the ear, we experience hearing. In the second instance
the stimulus, different sounds, is open to much greater
variety than in the first where the stimulus is, a deform-
INTERRELATION OF THE CHEMICAL SENSES 177
ing pressure, and in a corresponding way the sensations
in hearing are much more diverse than those in touch.
But it is still reasonable to regard hearing as one sense,
for its various stimuli grade into one another as its sen-
sations do. With taste on the other hand such is not the
case. The acid stimulus as an external agent is entirely
distinct from the stimuli for the other tastes and the
sour sensation as an internal state does not grade into
other gustatory sensations. This separateness in stim-
uli and in sensations is characteristic of the four kinds
of tastes and justifies their acceptance as separate senses,
a division that is not permissible in hearing. To con-
stitute a single sense implies a reasonable similarity in
stimulus, receptive mechanism, and sensation.
But, as previously pointed out, the initiation of sen-
sations is a function of only a limited number of the
human receptors. Many of these organs are concerned
with activities entirely unassociated with sensation;
hence to speak of them as representing a sense seems
somewhat inconsistent. If the term receptor is an im-
provement over that of sense organ because of its free-
dom from implications concerning sensation, it might be
well for the same reason to substitute some other term
for sense, such, for instance, as recept. 1 In that case a
recept is that aggregate of action that occurs where the
receptive arm of any reflex arc goes into normal activity
irrespective of whether this activity is productive of a
sensation or not. The recept then includes all the oper-
1 1 am fully aware that this term has already been appropriated by
the psychologists for a very different purpose, but as they have taken
almost all the terms in the language for their own use, I do not hesitate
to reappropriate this one to fill the present need.
178 SMELL, TASTE, ALLIED SENSES
ations from the reception of the stimulus to those central
changes that mark the entrance of the impulse into the
central organ including the production of a sensation,
if such occurs.
Where a recept is concerned with sensation, the pro-
duction of this state may be regarded as its final step. A
sensation, then, is an activity in a particular region or
spot of the central nervous organ marking the central
end of the receptive portion in a reflex arc. Experience
has shown that, irrespective of the means by which this
central region is stimulated, it calls forth only one kind
of sensation. This in a way is a restatement of the mod-
ern view of Miiller's specific energy of the nerves, for,
according to this principle, however a particular sense
organ, or conducting trunk, or nerve center may be stim-
ulated, only one kind of sensation results. In other
words the character of a sensation is not determined by
peripheral organs but is strictly a central affair and sen-
sations are different not because of the different sources
of the incoming impulses, but because of the different
central spots excited. Since the anatomical connections
are such that a particular receptor always leads to a
special central region, it follows that such a receptor be-
comes thus associated with a given sensation. Hence
where sensations occur they may be used in distinguish-
ing receptors, but in the many recepts that are unassoci-
ated with sensation this feature naturally cannot be called
upon as a means of discrimination.
Although numerous receptors are in no way concerned
with sensations, there are no receptors that are not ac-
tuated by stimuli. Hence the stimulus affords a more
general basis for discriminating between receptors than
INTERRELATION OF THE CHEMICAL SENSES 179
the sensation does. The two groups of chemoreceptors
and of mechanicoreceptors, already frequently alluded
to, show how fundamental this method of classification
is, for these two groups represent the two well-recognized
activities of our material surroundings and together may
be put in strong contrast with radioreceptors such as the
organs for heat and for cold and the eye, all of which are
stimulated by radiant energy.
These three classes constitute the fundamental groups
of receptors and under some one of these heads every
such organ should find its place. To the chemoreceptors
discussed in this volume may possibly be added those
on the wall of the stomach that according to Carlson
(1916) have to do with appetite. The receptors for pain
are possibly stimulated by the chemical action of ab-
normal tissue juices and the endings for thirst may also
depend upon some such form of activation (Cannon,
1918), though both of these organs may belong to the
group of the mechanicoreceptors (Muller, 1920). To the
mechanicoreceptors belong; unquestionably those termi-
nals that are excited by a deforming pressure such as
the receptors for touch, for pressure, including the or-
gans for equilibrium, and for hearing. Very probably
pressure is the stimulus for muscle, tendon, and joint re-
ceptivity and the sense of fullness in cavities. Pressure
produced by the contraction of the muscular walls of
the stomach appears to be the stimulus for the hunger
pang (Cannon and Washburn, 1912). The lateral-line
organs of fishes and amphibians give every evidence of
being* mechanicoreceptors. Finally radiorecepto-rs are
those organs that are stimulated by radiant energy such
as the heat organs, the cold organs, and the eye.
180 SMELL, TASTE, ALLIED SENSES
To ascertain into which of these three groups a re-
ceptor falls it is necessary to know how it is stimulated
after which its classification is simple and immediate.
Although a grouping of receptors based upon their
stimuli will of necessity always be complete, this plan of
arrangement is not entirely devoid of difficulties. Chief
among these is the fact that the same stimulus may ac-
tivate what we know from other standpoints to be differ-
ent receptors. Thus, as already stated, parabrombenzoic
sulphinide excites sweet receptors as well as bitter ones,
and strong material vibrations will stimulate the organs
of touch as well as the ear. But such instances appar-
ently occur only between closely related receptors, for
the organs for sweet and for bitter are so closely related
as to be regarded by many as belonging to one category
and hearing is certainly very near akin to touch. Herrick
(1918) has discussed the definition and classification
of receptors and has urged for this purpose the use of
four criteria : the sensation, the stimulus, the sensory
mechanism, and the type of response. In his opinion,
however, none of these affords a wholly satisfactory basis
for discrimination and grouping, operations that can be
successfully carried out only when sufficient information
is at hand. But experience scarcely warrants such a
conclusion, for it is much more difficult now to discover
the interrelation of the twenty or more human receptors
with all that is known about them than it was to make a
corresponding statement about the original five. The real
difficulty lies in the fact that the numerous receptors that
we now recognize have undergone varying degrees of
differentiation and hence their mutual affinities are ex-
tremely diverse. This brings us at once face to face with
INTERRELATION OF THE CHEMICAL SENSES 181
one of the problems of this inquiry, namely, the genetic
relations of receptors.
5. Genetic Relations of Chemoreceptors. The three
sets of receptors mentioned in the last section, the
chemoreceptors, the mechanicoreceptors, and the radiore-
ceptors, are more than mere convenient assemblages ; they
represent natural groups of organs whose relations with-
in each group have a certain genetic character. This can
be illustrated by the chemoreceptors.
Fia. 37. Diagrams illustrating the receptor systems of the following verte-
brate chemoreceptors: a, olfactory organ and vomero-nasal organ; b, organ of the
common chemical sense; c, gustatory organ. After Parker, 1912.
If the structure of the several vertebrate chemore-
ceptors is compared, it will be found that they present
three types of organization (Fig.37). These types can
be best appreciated from the standpoint of their constit-
uent neurones. In the olfactory and vomero-nasal
organs the neurones have cell bodies in the receptive
epithelium and their axons extend as nerve-fibers from
these bodies into the central organ. In the common chem-
ical organs the receptors are free-nerve terminations in
the mucous epithelium of the mouth, the nose, the eye and
other such apertures, from which axons provided with
182 SMELL, TASTE, ALLIED SENSES
deep-seated cell-bodies extend into the central organs.
Finally, in the gustatory organs the taste-buds are com-
posed of receptive epithelial cells that are in synaptic
relations with nerve terminals essentially like free end-
ings from which axons with deep-seated cell-bodies pass
into the central organs. These three types of structure
include, so far as is known, all the vertebrate chemorecep-
tors. To a common stimulus, like ethyl alcohol, the ol-
factory type has been shown to have by far the lowest
threshold followed in order by the gustatory and the com-
mon chemical types both of which are near together in
When these three types are compared with the recep-
tors of other animals, it is seen that the olfactory type
reproduces almost exactly that found in the skins of many
invertebrates, and that the other two types are character-
istically vertebrates. The integument of animals even as
simply organized as sea-anemones is rich in receptive cells
that reproduce in almost eveiy detail the conditions of
the vertebrate olfactory neurones. Not only do these
lowly organized forms show this structural similarity in
their integumentary cells, but they are known to be so
responsive to minute amounts of material wafted from
distant food through the water to them that they have
been for a long time past credited with olf action (Pollock,
1883). Thus the vertebrates olfactory epithelium and the
integument of aquatic invertebrates are strikingly alike.
It is more than probable that the vertebrates have
descended from ancestors whose skin was an epithelium
like that on the exterior of a sea-anemone and that, as
this skin thickened over most of the body to give the
necessary protection to the slowly metamorphosing ani-
INTERRELATION OF THE CHEMICAL SENSES 183
mal, the future olfactory region remained unchanged and
thus retained its original invertebrate character. This
region became the olfactory epithelium of the developing
vertebrate, the most primitive chemoreceptor in this
group of animals.
The organs next in this series were the common chem-
ical receptors. The neurones for these organs were
differentiated from the neurones of the primitive inverte-
brate skin by a central migration of their cell-bodies till
they became part of the spinal ganglia and thus left in the
integument free-nerve terminations as receptors. This
type of chemoreceptor is found generally in the skin of
fishes and amphibians and in the mouths, nasal chambers
and other moist cavities of the air-inhabiting vertebrates.
The third and last type of the vertebrate chemorecep-
tor is the gustatory organ. In this type the conducting
neurone presents exactly the condition met with in the
common chemical receptor excepting that its nerve ter-
minals, instead of being free in the integument, are asso-
ciated with epithelial taste-buds. This type of receptor
was probably derived from the second type by the appro-
priation of taste-cells from the integumentary epithelium.
Thus the three types of vertebrate chemoreceptors
appear to be genetically related in that the olfactory
organs represent what may be called the first generation,
the common chemical the second, and the gustatory the
third (Parker, 1912).
But within each type much detailed differentiation
has taken place. It seems to be quite impossible to ex-
plain the variety of olfactory sensations without assum-
ing a differentiation among the receptors of the olfactory
field. In the common chemical sense the receptors on
184 SMELL, TASTE, ALLIED SENSES
the moist surfaces of the eye, judged by the sensations
they give rise to, are distinguishable from those in the
epithelium of the mouth and of the nose. But this special
differentiation is best seen in the gustatory organs. Here
three and probably four well defined senses can be dis-
tinguished, namely, sour, saline, sweet, and bitter. And
though separate receptors for these four senses have not
as yet been distinguished structurally, their functional
separation is beyond doubt.
It is because of the repeated differentiations that
characterize the evolution not only of the chemoreceptors
but of the other groups of like organs that a classification
of them or even a simple enumeration proves to be so
unsatisfactory. For they are not unitary elements that
can be counted like the fingers on the hand nor are they
sufficiently co-ordinated to make classifications easy and
natural. They are like the whole organism itself in that
they exibit that kind of diversity that characterizes evo-
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CANNON, W. B. 1918. The Physiological Basis of Thirst. Proc. Roy.
8oc., London, B, vol. 90, pp. 283-301.
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sinnes. ftitzb. phys.-med. Soc., Erlangen., Heft 18, pp. 7-10.
FLOURENS, M. P. 1828. Experiences sur les canaiix semi-circuluires de
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INTERRELATION OF THE CHEMICAL SENSES 185
VON FREY, M. 1896. Untersuchungen iiber die Sinnesfunotionen der
menschlichen Haut. A bh. Sachs. Gesell. Wissensch., math.-phys., Cl.,
Leipzig, Bd. 23, pp. 169-266.
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HERRICK, C. J. 1918. An Introduction to Neurology. Philadelphia and
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Menschen, Bd. 3, pp. 109-282.
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Acetic acid, 137
Activators, 18, 21
Adequate olfactory stimuli, 80
Aducco, V., et U. Mosso, 153, 156,
Alcock, N., 33, 39
Alkaline taste, 134, 135
Allison, V. C., and S. H. Katz, 52,
55, 56, 88
Althaus, J., 81, 88
Amiurus, 116, 160, 173
Anosmia, 71, 72, 83
Von Anrep, B., 153, 162
Anton, W., 92, 100
Arnstein, C., 29, 39, 121, 122, 128
Aronsohn, E., 57, 58, 60, 63, 70, 81,
83, 85, 88
Asai, T., 33, 34, 39
Babuchin, A., 29, 39
Backman, E. L., 60, 61, 77, 88
Baginsky, B., 125, 128
Baglioni, S., 64, 68, 88
Ballowitz, E., 31, 39
Barral, F., et A. Ranc, 162
Basal cells, 29, 30, 119
Beaunis, H., 155, 162
Becker, C. T., und R. O. Hertzog,
Bedford, E. A., 39
Beer, T., A. Bethe, und J. von
Bethe, A., 17, 21, 22
Bidder, F., 48, 87
Bishop, J., 43, 88
Bitter taste, 140, 150
Blakeslee, A. F., 72, 88
Blaue, J., 34, 39
Blix, M., 175, 184
Bowden, H. H., 39
Braeuning, H., 108
Broman, I., 99, 100
Brookover, C., 98, 100
Von Brunn, A., 28, 29, 31, 34, 35, 39,
96, 97, 98, 100
Bulbar fibers, 123
Bunzel, R., 147, 164
Cannon, W. B., 179, 184
Cannon, W. B., and A. L. Washburn,
Carlson, A. J., 179, 184
Castronovo, A., 29, 33, 34, 40
Chemical relations of odors, 76
Chemoreceptors, 169, 175, 176, 179
Chorda tympani, 126
Circus movements, 67
Classification of receptors, 176
Coghill, G. E., 106, 108
Cohn, G., 78, 88, 141, 144, 162
Cold organs, 179
Cole, L. W., 103, 107, 108
Common chemical organs, 181
Common chemical sense, 102
Comparative distribution of taste-
Comparative physiology of taste, 160
Component theory of taste, 152, 158
Concha?, 23, 27, 38 Gawrilenko, A., 93, 100
Copeland, M., 66, 68, 88 Gemmal fibers, 123
Crozier, W. J., 103, 104, 107, 108, Genetic relations of chemoreceptors,
138, 139, 158, 162, 163 181
Gushing, H., 125, 126, 128 Gertz, H., 147, 163
Glaser, 0., 71, 89
Diemyctylus, 68 Gley, E., 162
Disse, J., 34, 36, 39, 40 Gley, E., et C. Richet, 141, 163, 170,
Distribution of taste, 148 185
Dogfish, 64, 173 Glucophore, 144
Dogiel, A. S., 33, 40, 115, 123, 128 Goldfish, 63
Drasch, 0., 125, 128 Golgi, 32
Drugs and taste, 152 Goldscheider, A., und H. Schmidt,
Duges, A., 43, 88 151, 163
Durand, A., 48, 60, 68, 88 Graber, V., 15, 22
Durrans, T. H., 88 Graberg, J., 110, 114, 116, 118, 120,
Duval, 134, 163 12 8, 129
Grassi, V., und A. Castronovo, 29,
Von Ebner, V., 116, 128 33> 34; 40
Ecker, A., 28, 40 Greenberg, D., 69, 91
Eckhard, C., 28, 31, 40 Group of chemical receptors, 174
Edgeworth, 152 Gustatory chiasma, 125
Ehrlich, P., 29, 40 Gustatory contrasts, 156
Electrical stimulation of taste, 147 Gustatory nerves, 123
Eschricht, D. F., 43, 89 Gustatory nerve fibers, 127
Ethmoid cells, 26 Gustatory organs, 110, 182
Extrabulbar cells, 120 Gustatory senses, 158
E y e 179 Gustatory stimuli, 133
Fischer, E., und F. Penzoldt, 53, 54, Gvmnemic acid, 153
56, 89, 184
Flourens, M. P., 174, 184 Hahn, R., 132, 164
Foliate papillae, 113 Haller, B., 73, 115, 129
Fontana, A., 153, 163 Hamlin, H. E., 100
Franke, G., 45, 47, 89 Hammerhead shark, 67
Free-nerve endings in olfactory re- Hiinig, D. P., 132, 149, 163
gion, 34 Harvey, R. B., 138, 163
Frentzel, J., 156, 163 Haycraft, J. B., 57, 77, 87, 157, 162
Von Frey, M., 54, 89, 135, 163, 175, Hearing, 176
185 Heat organs, 179
Frolich, R., 44, 86, 89 Heidenhain, M., 113, 115, 116, 117,
Frontal sinus, 26 119, 129
Fungiform papilhe, 112, 150 Heiderich, F., Ill, 114, 129
Fusari, R., et A. Panasci, 121, 128 Henle, J., 136, 163
Kenning, H., 60, 68, 69, 74, 75, 76,
Garman, S., 89 78, 79, 80, 82, 87, 99, 100, 158, 163
Gaupp, E., 100 Henry, C., 163
5 el VT'i L '' I 41 '/ 55 ' 163 Kahlenberg, L., 135, 136, 137, 138,
Herhtzka, A, 135, 140, 142, 153, 139, 140? 164
154, 163 Kallius, E., 31, 34, 40, 100, 116, 129
Hermann, F., Ill, 118, 119, 120, 129 Kamon, K., 34 40
Herrick C. J., 106, 108, 115, 124, Karpman, B., 49, 91
129, 160, 161, 163, 172, 175, 180, Kastle, j! H., 154, 164
Hertzog, R. 0, 138, 162 g^ ** ,f ,f 56 > 88 .
Heymans, a, 156, 163 ^ ?, 136, 139, 140,
Hisiology of vomero-nasal organ, 96 3' } ^ 153 ' 155 ' 157 ' 158 >
' 135 ' 139 ' Kieso^ F, und R. Hahn, 164
A 11, 129
Hofmann, F, und'l, Bunzel, 147, ^^ 153,? 64
Honigschmied, J., 124, 131, 155
Hooper, D, 153, 164 Krause, W., 28, 40, 114, 126, 129
Howell, W. H, and J. H. Kastle, Kremer > J- H., 62, 89, 157, 164
154, 164 Von Kries, J., 175, 185
Hunger, 179 Landacre, F. L., 128, 129
Huyer, C., 78, 89 Larguier des Bancels, J., 61, 87, 162,
Inadequate gustatory stimuli, 145 Larsell, 0., 100
Inadequate olfactory stimuli., 80 Latency of taste, 154
Innervation of taste-buds, 120 Lateral-line organs, 15, 179
Insipidity, 135 Von Lenhossek, M., 34, 40, 97, 98,
Inspiration, 48 100, 118, 120, 121, 123, 129, 130
Integumentary sense, 175 Leydig, F., 16, 115, 117, 130
Intermediate zone, 32 Lingual nerve, 125
Interrelation of the chemical senses, Linnaeus, 73
167 Location of taste, 132
Intrabulbar fibers, 122 Loeb > J -> 62 > 89, 108
Intragemmal fibers, 122 Loeb ' R - F -> 62 > 89
Intragemmal spaces, 120 Loven, C., 110, 117, 130
Irritants, 44 Lubbock, J., 15, 22
Lucas, K., 13
Jacques, P., 121, 129 Luciani, L., 68, 87, 158, 162
Jacobson cartilage, 98
Jagodowski, K. P., 32, 33, 34, 40, McCotter, R. E., 94, 101
62, 89 Magendie, F., 43, 89
Johnston, J. B., 115, 129 Marchand, L., 162
Jourdan, E., 15, 22 Maxillary sinus, 26
Merkel, P., 118, 130
Metallic taste, 134
Meyer, S., 125, 127, 130
Von Mihalkovics, V., 99, 101
Minimum olfactory stimulus, 49
.Merrill, A. D., 33, 34, 40
Mosso, U., 153, 156, 162
Miiller, Johannes, 68, 152, 178
M filler, L. R., 179, 185
Munch, F., 115, 130
Myers, R. G., 144, 145, 164
Xagel, W., 48, 63, 64, 67, 76, 87,
89, 103, 109, 156, 158, 162, 164
Nasal cavities, 23
Nasal membranes, 26
Nasolacrimal duct, 26
Nerves of ol faction, 42
Nerve terminals of common chem-
ical sense, 104
Nettesheim, K., 143, 165
Neutralizing odors, 85
Odor mixtures, 83
Oehrwall, H., 135, 150, 151, 157,
158, 160, 164, 176, 185
Oertly, E., and R. G. Myers, 144,
Olfaction and radiation, 62
Olfaction in fishes, 63
01 faction and solvents, 60
Olfactory acuity, 53, 77
Olfactory cell, 30
Olfactory cleft, 25
Olfactory epithelium, 27, 28
Olfactory fatigue, 69
Olfactory flagella, 32
Olfactory hairs, 31
Olfactory nerve, 35, 42
Olfactory nerve fibers, 29
Olfactory organ, 23, 36, 181
Olfactory organ of fishes, 37
Olfactory potency, 77
Olfactory prism, 75
Olfactory reflexes, 86
Olfactory sense buds, 34
Olfactory stimulus, 57
Olfactory vesicle, 31
Olmsted, J. M. D., 68, 89, 127, 130
Organ of Jacobson, 92
Osmophoric groups, 78
Panasci, A., 121, 128
Papillae of tongue, 111
Parabrombenzoic sulphinide, 154
Parker, G. H., 22, 66, 89, 90, 103,
104, 106, 109, 161, 164, 173 183,
Parker, G. H., and E. M. Stabler,
55, 141, 143, 165, 171
Parker, G. H., and A. P. Van Heu-
sen, 148, 165
Passage of air through nasal cavity,
Passy, J., 54, 55, 77, 90
Paulsen, E., 45, 46, 47, 48, 90
Pawlow, J. P., 86
Penzoldt, P., 53, 54, 56, 89
Peribulbar fibers, 122
Perigemmal fibers, 122
Peter, K., 40
Physiology of gustation, 132
Physiology of ol faction, 42
Picht, P., 43, 90
Pier cells, 118
Tiutti, A., 133, 165
Pollock, W. H., 182, 185
Polymorphic cells, 33
Ponzo, M., Ill, 130, 165
Potassium chlorate, 156
Prins, H. J., 90
Qualities of odors, 72
Qualities of tastes, 134
Ramon y Cajal, S., 29, 34, 40, 97,
Ranvier, L., 118, 120, 125, 127, 130
Read, E. A., 40, 97, 98, 101,
Reflex action, 16
Retzius, G., 29, 31, 34, 40, 41, 97, g 143
"I/~\T i i f\ i n i 1 rr* i c\ o io/\ O '
Sternberg, W., 87, 134, 154, 162, 165
Stich, A., 134, 165
Stimulation of chemoreceptors, 167
Substances with two tastes, 153
101, 119, 121, 122, 123, 130
Reuter, C., 72, 90
Ribot, T., 185
Richards, T. W., 136, 137, 165
Richet, C., 141, 163, 170
Risser, J., 68, 90
Rod cells, 118
Rollett, A., 72, 90
Rosenthal, J., 146, 147, 165
Rubaschkin, W., 35, 41
Saline taste, 139, 150
Sandmeyer, W., 125, 127, 130
Supporting cells, 117
Sustentacular cells, 29, 30
Sweet taste, 142, 150
Symington, J., 101
Systems of odors, 73
Taste alteration, 155
Taste compensations, 157
Taste-bud, 110, 115
Taste cells, 117
Taste mixtures, 157
Sarasin, P., und F. Sarasin, 100, Thorns, H., and K. Nettesheim, 143,
Schaeffer, J. P., 24, 25, 41 Touch, 176
Schiff, M., 43, 90, 134, 165 Tourtual, C. T., 57, 90
Schirmer, R., 155, 165 Trigeminal nerve, 42
Schmidt, 151 True odors > 44
Schultze, M., 28, 29, 30, 31, 32, 41, Tuckerman, F., 110, 111, 115, 131
58, 90, 175, 185 Von Uexkiill, J., 64, 90
Schulze, F. E., 115, 130 Urbantschitsch, V., 165
Schwalbe, G., 110, 114, 117, 118, 130 Valentin, G., 43, 53, 81, 85, 90, 91,
Sense organs, 13, 18, 21 134, 165
Seydel, O., 100, 101 Van Dam, C., 51, 91
Sheldon, R. E., 64, 66, 90, 103, 104, Van der Stricht, O., 31, 41
Sherrington, C. .S., 172, 185
Shore, L. E., 149, 153, 165
Smell and taste, 173
Sour taste, 136, 150
Specific energy of nerves, 152, 178
Sphenoidal sinus, 26
Stabler, E. M., 55, 106, 141, 143, 165,
Stahr, H., 110, 111, 131
Steiner, J., 64, 90
Van Gehuchten, A., 35, 41
Van Heusen, A. P., 148, 165
Vaschide, N., 58, 60, 91, 162
Vastarini-Cresi, G., 125, 131
Vomero-nasal organs, 92, 181
Veress, E., 58, 59, 60, 81, 91
Von Vintschgau, M., 87, 131, 134,
Von Vintschgau, M., und J. Honig-
schmied, 124, 131, 155, 165
Washburn, A. L., 179, 184
Weber, E. H., 57, 58, 59, 91
Winslow, C.-E. A., and D. Green- Zenneck, 134, 166
berg, 69, 91 Von Zevne k, R., 147, 166
Von Wittich, W., 154, 165 _
Woodrow, H., and B. Karpman, 49, Zu <* erkandl > E - "1
91 Zuntz, N., 156, 166
Wundt, W., 134, 166 Zwaardcmaker, H., 45, 47, 48, 50,
Von Wyss, H., 114, 131 51, 57, 58, 59, 62, 70, 72, 73, 76,
Zander, R., 124, 131 77, 85, 87, 91, 162